Bf mom is narcissistic?

So im back with my ex bf whos mom is narcissistic and of course has cancer which i understand but the reason i had moved out was because she had gotten to my head with alot of stuff .now that im back in her son life shes starting up again i love her son to def and he told me that if i am pregnant that hes gonna have to take a dna test cuz his parents and sister .ive told him im positive i cant have kids atm from the times ive tired so im not to worried about that rn im just more stressed cuz of his mother ... So now i have to do things when shes up in her room at night which i dont mind ..i just need some way to just ignore her and tell her what she wants to hear ..