That's a good question. Clearly she does not give two hoots about damaging a girl's life or her reputation. Precisely what she wants to do, score and secure revenge for the public humiliation she supposedly suffered during purvi's marriage.
She clearly does not care of her son's happiness or respect his choices or decisions... she is trying to get what she wants another subservient DiL. See the pattern here, its all about 'her'. UB is selfish beyond words and that's what it all comes down to irrespective of what she says or cloaks it with... Tradition...culture ... She is so wrapped up in herself that everything invaribly becomes about her.
So the 2nd marriage, she is not thinking about her son and the emotional duress he would be under. It's still not about him.