Hindu marriage in relation

She cannot marry as she has common ancestor.  Such come under sapinda relationship and is prohibited under Hindu Marriage Act.

Save the nation by not marrying and producing genetically defective children and all headache of looking after such children in the future.

Brother sister marriage is not allowed in any religion.  

Hope you understand.

Counting the generation of the girl and the boy as one they have common ancestors 5 generations back. They can marry. In Maharashtra and Southwards even first cross cousins can marry.

Hindu can not marry if their relation falls in the Spinda relationship.

"Sapinda relationship" with reference to any person extends as far as the third generation (inclusive) in the line of ascent through the mother, and the fifth (inclusive) in the line of ascent through the father, the line being traced upwards in each case from the person concerned, who is to be counted as the first generation.

Two persons are said to be "sapindas" of each other if one is a lineal ascendant of the other within the limits of "sapinda" relationship, or if they have a common lineal ascendant who is within the limits of "sapinda" relationship with reference to each of them.

In my opinion this relationship may not fall undr sapinda relationship, hence, if the tradition and customs permit, there can be no problem in both marrying each other.