Gifts U/s. 56(2)(vi)

Although monogamy is norm, it is not absolutely universal. Think about it, you have the answer.

Please read the Section again and you will get it. For your reference relative means

1. Spouse of the individual.
2. Brother or sister of the individual.
3. Brother or sister of spouse of the individual.
4. Brother or sister of either parents of the individual.
5. Any linear ascendant or descendant of the individual.
6. Any linear ascendant or descendant of the spouse of the individual.
7. Spouse of the persons referred in clauses (ii) to (iv).

But yes defination of relative is a joke.

Take the instance of Miss A whose mother's brother is Mr. Y. Yes, Y is a relative of A(Mother's brother) but A is not a relative of Y (sister's daughter). I thought that is A is a relative of Y. the Y is also a relative of A.

It is quite possible for  the spouse to  have children from a different spouse  to whom he/she was married and subsequently got divorced or widowed   before getting married to the present spouse.  Viewed from this angle, the section makes sense.