Interesting topic and good question you asked. @ TM
Whether I'm right or wrong I don't know. I just got the feeling that Biji knows Balraj is terrible and won't spare anyone even his own kids if anyone turns against him.
So she directly speaks to Balraj, confronts him, advises him but that is all she does. She also observes what is happening around her as much as possible.
She thinks if she starts talking to family members or even M, they may talk directly to Balraj or give away from the way they behave that they know about his secret mission which may alert Balraj. He'll defeat that person somehow. Basically she does not think anyone has the ability to tackle him.
Her being cautious is v. valid point as we saw the angry Nandini was easily defeated by his magic words today 7th May. Supposedly intelligent and independent girl Meera was swept off by Balraj's emotional blackmail and sweet words.
These two girls just don't know how deep Balraj's mind is but Biji does know. Based on her experiences from the past , she does not think it's right time to let rest of family or M or N know about Balraj's actual plan.
Balraj is shown so far as way too quick thinking person for anyone member of Aasthanas or collectively.
I think, Nandini's dad's entry may give a way for a solution so that at least N will come out of this Balraj's caring act. I just don't know how Meera will come out. Something has to happen in the house to trigger Meera's old self. Agasthya so far shown as loyal to Balraj, even if he suspects M is in danger, I don't know how exactly he can help her. Lets see.