How can I marry the niece of my cousin sister

You are in love with your cousin's sister's niece. There is nothing wrong in it but when the question of marriage comes then the senior members in the families have a say and many times if they do not agree then it becomes a difficult proposition for the boy and the girl to make them understand about their love and the commitments that they are having. You have to do an analysis of the situation before requesting your parents or requesting directly her uncle about this issue and try to get it resolved.
First thing is whether you are working or unemployed. This is a point on which many marriage proposals are ruined quickly. What about the girl? Is she employed or unemployed? The question that the parents or guardian often ask is what would be the source of livelihood after the marriage. If both of you are going to be dependent on your or her parents (uncle) then it would not be a proper thing to do in todays inflation prone environment. So if you are unemployed then first make a career and then only think of the marriage. In case you are employed then what is your position in the industry especially with respect to the girl's family and their financial conditions. This is a very important point because generally the parents will like to have relationships in the same financial levels to avoid any embarrassment in the future.
Another important aspect is of compatibility. In the young age there is often a infatuation of young minds and they start loving a partner but in real life when the time comes to adjust and accommodate then they entirely fail and the relationship breaks. So believing that all that glitters is not gold one has to go deeper and find out that what would be the attitude of the other person if there are tough times like lack of finances or lack of physical facilities in their lives. The real nature would only emerge under adverse life conditions. The infatuation will die soon but real love remains for lifetime.
One more thing that you have to find out and ponder, may be after talking to the girl, that suppose her uncle does not allow her for this marriage but she goes for it and leaves him and he also disowns her then can she survive without her uncle in her life later. What is her confidence level for that? It is necessary to know that because if she is not having any confidence in displeasing her uncle and going ahead with this relationship then case gets closed at the present unresolved state. Will your family be ready to bring her home in that situation? That is also to be ascertained by you by talking to your parents.
If you think about all these aspects then it would be easier to take a decision in this matter.

Basically, the problem you are facing is that you are not getting the nod from the family of the girl.
Another point you have mentioned is that you can't marry without the consent of her family.
You have not mentioned the reason for her uncle not accepting the proposal.
1. Both you and the girl should sit together and discuss the points her uncle is mentioning for not accepting the proposal and how important they are. If you can get some supporting points to satisfy her family members which make them accept your proposal.
2. The girl should discuss all the points, in support of her accepting you and why she wanted to marry you, with her family members. She should bring a feeling in the minds of her family members that she may go to any extent to marry you. Once they get convinced that they can't stop her from marrying you, there will be a change in their thinking process definitely. if she is requesting them to consider the case and if they are yielding, your would-be should open up her mind to at least one of the members of her family and tell her that she made up her mind that under any circumstances, she wanted to marry you. Then only there may be a positive movement from their side.
3. Once she completed her exercise your family members can meet her family members and discuss with them your decision also. They should explain that both of you are how strongly decided to go ahead and also tell them that they are in agreement with the proposal. They should also mention that irrespective of the elders' consent both of you may go ahead which may not be good for both the families. Hence it is better for the elders to accept their proposal.
Once her uncle gets messages from both the parties involved if may get pressurised and may change his opinion and may give his acceptance.
4. Even after all this if her uncle is not willing to the proposal, you both should ignore him and go ahead. I think both of you attained the age of a major. Both of you should get the marriage done in the registrar's office. If any problems are encountered you have to take the help of the local police.
But give sufficient time to think and decide on the course of action. Before going ahead you have to think about the financial implications also.

If you have taken a decision of going ahead with the alliance with the niece of your cousin's sister, there is no one outside opposing your decision of such a companionship. However, you have to ensure yourself how much comfortable you are with your family in taking such a crucial decision apart from psychological impact of your fiancé due to her excess stress development due to clashes of her views with her family members. It would be a difficult situation at least for the time being for both of you till the same is resolved with your mutual decisions in settling your marriage.
However, you cannot go ahead unless there is the frank and open discussion between you and your would be spouse taking all the vital points connected with the settlement of the marriage.
So far so good with the prudent planning but here the matter does not end unless you get a final consent from your in-laws side especially the mother - in - law and father - in - law and without taking them into confidence, your marriage is not going to work out.
Taking both the families- yours and yours partner's family would be much better in expediting the process of marriage.
If that situation fails, both of you can meet a lawyer to work out a feasible solution culminating in the settlement of the marriage in the court. This would not involve major expenses other than inviting ten prominent people of both the parties who would even be the witness of the marriage and would also grace the occasion offering you blessings.
In the meanwhile understand your partner completely so that the relationship does not get strained unnecessarily in future.

Nowadays love marriage is very common. I think in the present scenario arranged marriage has become risky. As mentioned above, you are in love with your cousin's sister's niece. So, your marriage would come under distant relative. But there is no problem. Your main problem is that you are not getting permission from her uncle. In my view, at first, both of you should try to convince him as far as possible. And after trying hard if they are not agreed then both of you should do court marriage. Here, you did mention your financial status. In today's life, there is no importance of relationships. You must keep in mind that money is all in all. Both of you are working or not. If anyone is working then no problem. Both of you can manage up and down of life. If both of you are unemployed then after marriage you all will have to face a lot of problems and your marriage would be hell. So, If you are not working then your first work is to search for a job and then think about marriage.

I have read the question. The relationship as mentioned by you does not come under prohibited relations. So, if both of you are adults, you can marry. If social marriage is not possible due to objection from the girl's relative, you may go for registry marriage. Such marriage is safe to avoid any possible dispute in future from any relative.

If there is no common immediate ancestor, then there is no problem in your marriage. In this case it does not appear to have any such common ancestor. The question does not give those details, but on a preliminary deduction it is presumed there is no immediate common ancestor. However this may be clarified to ensure that the marriage is not legally or socially prohibited.
Thus assuming, the suggestions are given bellow:

1. As your parents have agreed for the marriage half the problem is already solved.

2.Ensure that girl's mother does not have any objection.

3. Discus with her uncle and ascertain why they are against this marriage. Then convince them and remove their doubts by establishing the justification for the same. Take suitable remedies for them if not already taken. Also assure them that she will be happy and safe with you after marriage.
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4. If your cousin is in good terms with both sides, use the cousin's goodwill for making things favourable by needed discussion and intervention.

5. If still her people are adamantly against the marriage, tell them clearly that you will proceed with the marriage even without their permission. For this you need the wholehearted support from the girl. She may still have some sentimental attachments to them as they had taken care of her. Then with the support of your family, her mother's best wishes and support and with the help and presence of well wishers, proceed with the marriage.

My Best Wishes.