About getting married with relative girl

Your mom's cousin brother's cousin sister will be your mom's sister only. So that girl will be your sister relation. But it is a very distinct relation. There are instances which I know like this. My uncle's(father's brother) wife is aunt to me. She is having the same relation with my wife also. She is aunt to my wife. But the relation is not very close. So we need not worry much about that.
But all depends on the individual and his family. My personal opinion is that you can go ahead. However you think properly and take the opinion of the girl. You also talk to your parents and her parents and then decided.
If you are in love with her I think you can go ahead.

Please be clear about relations you are saying. Cousin are actually paternal or maternal. So mom's cousin brother's means actually Mom's mother side or Father side is not clear. Anyways. It seems tree is from 3rd generation above you linkage. You are safe to marry her if alliance agreed by parents. Just make sure don't marry any one of direct siblings of parents. More genes diversified more your future generation get chance to be pure.

Cousin marriage in India differs with culture and region. You have neither mentioned your caste nor your native state. As per the Hindu Marriage Act that was introduced in the year 1955, marrying cousin is illegal in Hindu community. The only exception is given keeping into account regional customs.

Now, these rules are about marriage related to first cousins. Cross-cousin marriage is legal. In case you are marrying your second or third cousin who comes from your mother's side then the marriage will be considered as legal.

As I can understand the girl is your cousin. As Hinduism, you cannot marry a girl, if Gotra is the same. But according to the legal point of view, if you have completed 21 years and girl has completed 18 years you can marry her.
But this type of marriage is always a problem for the entire family. I will suggest you should try to convince the family members. After that take your decision wisely.

The relation you have indicated is not a near cousin (first or second) and so is acceptable for marriage even socially.

In some societies it is a social stigma to marry the near cousins. Earlier it was believed that marrying in close relations sometimes brings the genetic disorders as more profound in the offsprings. This theory is to little extent supported by the medical science also. Due to these reasons many people feel apprehended to marry in the close relations.

In some countries, marring in close relations was not socially permitted but people made it changed through legal ways and though still the close marriages are seen with social stigma, due to legal clearance some people are going for it.

Even in Indian society the marriages within near cousins are seen with social as well as medical concerns.

On the contrary, in some specific societies, marrying in close relations is vehemently permitted. There is no social taboo for this in those cultures.

Logically seeing, the evolution of the Homo sapiens must have been from a limited population and then there was no other alternative expect marrying in close relations. With the increase in population and formation of tribes certain moral attributes were formed and guidelines were proposed for marriages between the tribes or within the tribes. These later became the basic social frameworks and remained in the different cultures preserved in their original form.

Today, in the modern societies with the blessings of legal framework and lack of clarity from medical sciences the tendency of marrying in close relations has increased. From a rough estimate more than 10% of the marriages in the world are in close relations.

Marriage -negotiation with the closed ones is not preferred in the Hindu societies other than the South - indians where su h relationships are not considered as taboos
In fact, the reason for the same is the Aggravation of certain blood disordes which might take place during the birth of the child.
In your case, it is not applicable and hence you can proceef for marriage provided you have analysed the other factots such as coherent temperaments of both of you and no financial constraints are to develop as a result of such a companionship.