@airi0na
Likely because if he finds out it's his biological child, her peaceful isolated life with Niti will end.
She also, as is hinted in this chapter, is significantly depressed and likely has some form of PTSD from all those years of abuse, and add that in with guilt for subjecting her son to it as well, and she's pretty much a broken woman.
As a result, she likely doesn't trust him anymore than she trusts her own father.
Add in the fact that there's a rumor Baster is a murderer, and you can see why she wants to keep her distance.