Can i marry relative girl ( seems like cousin sister)

Yes, you can marry her since you are related to her in some way but in no way she is your first cousin and thus it is not going to cause any difficulty in your marriage. Since it is not a close relationship thus, you can marry her.

Involve your parents and talk through the whole matter before getting married. Cultures and traditions differ from one caste to another. Thus, you need to sort out lots of things including the marriage between cousins topic before tying the knot.

The relation is very distant and there will not be any problem. There are instances where such girl and boy married together. Anyhow before deciding on the marriage get the blood group of both of you tested. Marrying a person having the same blood group is not advisable.
Now take the opinion of the girl also. If she is also interested, your parents and her parents can discuss and arrange the marriage. If your parents' object or her parents object, you both people have to take a call and decide accordingly.

There are many societies where it is unethical to marry with near cousins. In your case it does not appear as a near cousin so you have open option to go for marriage or not.

The matter of marrying with near cousin is a controversial subject. There are some theories and to some extent the medical science also advocates against the marriage with near cousins. The reason attributable to this is the manifestation of hereditary diseases in the offsprings out of these marriages. It is not that all the children produced through these marriages will be having those ailments but medical science says that the chances are more for such dangers.

Due to this apprehension many people avoid marrying in very near relations.

Another thing to note in this context is that in some communities the marriage in close relations even with the near cousin is allowed. We have to find out what is the statistics of getting hereditary ailments in such cases. This is a subject of research and many scholars are on their way to find out the real truth out of the near cousin marriage.

Marriages between blood relatives can lead to many gestational and genetic problems causing different ailments to the children born out of such marriages. Therefore marriages of blood relatives are not advised.