Hindu marriages act

Yes its possible.

But its matter of two family lifes so don't take step at once.

think before take step.

seems its matter of love.

For your son your sister is Aunt (Pinni i.e next to mother). Aunt's daughter is sister to your son.

Mr. SSS and Ms. DDD are supposed to be cousin brother and sister. No law in Hindu marriage act will acept.

In south India it is common to marry father's - sister (atta) daughter.

This case is one more step further i.e. father's - sister's- sister in law's daughter.

There is no prohibition as per Hindu Law.

Mr. XXX and Mrs. XXX are husband and wife being married to each other. Your son Mr.SSS is nephew of Mrs. XXX and hence her blood relation. Ms. DDD is niece of Mr. XXX and hence his blood relation. Mr. SSS and Ms. DDD belong to two different families related to each other only by marriage (of Mr. and Mrs. XXX). Mr. SSS and Ms. DDD are not even remotely blood relations, unless there is some other connection. Hence their marriage is not prohibited under Hindu Marriage Act.