Very very confused for a long time about this marriage question

Actually, you will be an aunt to that boy. In Hindu families, this relation is not good for marriage. In our region also these marriages are not allowed.
Coming to the legality of the issue, this marriage is not legal as it will come under the sapinda relationship. The blood relation comes in the 4th generation starting from you from your father's side. But up to 5 generations this sapinda relation will extend from father's side. So legally it is not valid.
After getting married if any problem comes and anybody questions the legality of the marriage the courts will not approve your marriage. If nobody complains and if your elders approve of your marriage, you may not have any problem.
So you should think thoroughly from all angles and then only you should proceed. You have to discuss with your parents and the parents of the boy and convince them. Otherwise, after marriage, if any of them challenges the marriage court also may not support it.

The relationship you cited comes to the effect that you and the boy are second generation cousins. That is it can be generalised as brother-sister relationship. As far as possible this kind of marriages are avoided.

However in certain communities which follow the patriarchal lineage in tradition and practice, this may be acceptable.

Hence if the system is accepted and followed in your family and community and also in the place where you reside, and if there s no known defects or hereditary type diseases in the family,, and if you and the boy love each other and match otherwise also, then you may proceed with the marriage,

So it is suggested you discus and consult your parents and family elders and do accordingly.

The relationship between you and the girl can be termed as brother and sister relationships and such marriages are prohibited in our Indian Society except in a few places where such marriages have taken place. This being a Sapinda Relationship, the court even does not approve of it.
Now in your case, it will depend upon the custom of your society and the acceptance of both your parents and the parents from the side of the girl. If everything goes well between both the families of yours and girl's side, you will have to think over how much comforts you will get with such an alliance. It would be better on your part to enjoy the close relationship with the girl to assess her temperament suiting you for the final proceedings.
Take the advice of the medical experts so that genetical disorder does occur between you and the girl as a result of such a marriage. Both of you would undergo for the genetical test so as to avoid disorder.
Finally, you may tie your knot with the girl if every phase goes favourable.