Do I tell her she’s being or will be cheated on!?

So my boyfriend and his siblings won’t tell their mother that they found out their father is or is planning on cheating on their mom..

I’m so confused what to do.. the family already hates me and she especially already hates me, so is it even my place to tell her? I’m so confused why they won’t anyway... they are trying to say she knows .. but I think they are saying that to not feel bad about keeping it from her.

I feel it’s not my business nor my place to tell her... I feel she’ll just slap me and tell me to stay away or something.

FYI I’ve been around for about 2 years and she still treats me horrible.. his sis has told me to my face multiple times she doesn’t like me.. if they were nice to me I feel like this would be no question and i would have already talked to her.

I mean what would you do?