Rights of second wife children

Terminology used in this answer:

1st Wife = A

Husband = B

2nd Wife = Y

2nd Wife's son = X

I am answering your query by assuming that A died intestate (i.e. without any will) and all the parties in the case are Hindus.

Upon perusal of Sections 15 & 16 of the Hindu Succession Act, 1956, it becomes clear that the property of A will devolve to her daughters, son, and husband (B), in equal proportions.

Now, once B gets his share of the property, he can dispose of it the way he likes. He can sell it or gift it to X. He can also create a will in favor of X.

Though, Right of X to directly have a share of A's property, after her death, does not arise in the scenario mentioned by you.

Thank your Sir