cousin marriage

can i marry my third paternal .I am in deep love with her for more than 5 years but the south indian culture says she is sister to me but i cant agree that because they allow to marry in first cousin (cross cousin )though the hindu marriage act says it comes under the degree of prohibited relation but they are allowing that kind of marriage. but when compared to my third cousin we are geneticall seperated far, how they are saying she is sister to me i cant agree that genetically.can i do register marriage by converting into any other religion?