My mom (46 F) has sex with her brother-in-law (my uncle) (M 47) since after my dad's death (5 years).

My dad died when I was in high school in an accident. He wasn't a great father. Always made us struggle without things because he wouldn't buy them, but would give money to others which he could never collect back. He wasn't an ideal husband too - used to hit my mom sometimes when in too much anger, and made her struggle without giving any money.

I understand all of this. After dad's death, his brother was always there for us. He and his family helped us throughout. And in the process, he kind of convinced mom that he would love her, take care of her, blah blah, and started having sex with her.

I wasn't here the next year after dad's accident because I went and lived in a hostel for further education. My younger sister was the one who saw the weird equation between my mom and my uncle. They would both be happy at times, and sometimes mom would get angry at him for no reason. This was all based on whether they were being intimate or not, which we both soon realised and started to ignore whenever we saw something.

I stopped talking much to mom after this, because I didn't feel even a little good about it. She told me a year later and expected me to forgive her (which I did) but this is never-ending.

Mom keeps threatening us saying she has depression and that we'll all suffer after she's gone. She doesn't see how she's helping a married man with two young kids, cheat on his wife, by being the other woman.

I understand my mom's sense of loneliness and grief. But there are healthier ways to channel it. Cheating is not the answer. I don't want to talk to her rudely, but what can I do?

I found out they're still doing the same behind our backs and haven't changed a bit. The uncle is the one to blame but I can't argue or threaten him, because he might ask me to leave house or something (it's drastic but he might do). I've tried looking for therapists for my mom but couldn't find any one in my city who is affordable.

What could I do in this situation? Should I tell my aunt about this? How's a better way to go about it? I am still studying and have exams going on at the moment too.

TLDR: My mom cheats with my uncle on his wife, after my dad's death and I don't know what to do anymore.