The income tax law has a very narrow defintion of the term "relative" so as to disallow certain monetary transactions.
If you want to use the word relative in the context of Hindu Marriage Act for the sake of seeking divorce or contesting a divorce then there are two questions :
1. Is it for establishing that the marriage was illegal (between relatives) ? The list of such relatives is known on this forum. Please search. The marriage between relatives is not allowed, one of the parties can apply for nullifying the marriage.
2. Is it for establishing extra marital relationship that the person is living with a person who is not a relative ? This is left to interepretation by the courts. The norm is the same relationship that bans the marriage can be used to establish that the person does not have any illicit relationship. However for any other live-in relationships, it has to appear in the eyes of the society as a live-in relationship. Sharing an apartment for the sake of convenience is an example if everyone knows you are just sharing an apartment, will not call for alleging an illicit relationship.