I don't think Saroj is trying to separate MohUr. She just will never accept Kastur. Saroj so far doesn't really go out of her way to make it horrible for Kastur. Just that she ignores her as much as she can. And sometimes her ignoring Kastur is what got her into a bit of trouble with Baapji, explain to Kastur some of the ways of the house, the first time and she'd be clear.**
The idea that Mohan should stay out of Saroj and Kastur is something I agree with and disagree with. First of all, I don't think Saroj will actively be out to destroy Kastur. She will just continue doing what she's doing. But that means she will scold and find fault with Kastur whenever she has the chance without bothering to listening to Kastur.
Mohan does need to stand up for Kastur when it's needed. He did that by going to Saroj and explaining it was his fault. If a man can't stand up for you at the right times, he's not worth it. That's why I do think there his relationship with Saroj will be affected somewhat. Not completely, but it will be.
Because Saroj cannot keep on dealing with Kastur with hate in heart, and not do something that will make Mohan feel that he has to stick up for his wife.
**Though only five nights have passed since their marriage. Till coming back from Kastur's house, Saroj could be comforted in Mohan's indifference to Kastur. Mohan telling Baapji, why not have her stay for a few more days, etc. That made it easier for her. But then they come back, and he seems to share a better dynamic with Kastur.
So we have yet to see how Saroj will react to what she perceives to be change in MohUr's dynamic. And add to that all that other stuff that happened with Baapji and the Rangoli... Saroj might step up her negative behaviour.