Hmm the thing with therapy is not every therapist will be a good fit for you. You need to try several until you find the right one. Some therapists will make you feel relieved, and others will make you feel weird like "why should I share my secrets to you??".
If I was you I'd be far far far more angry at the sister. Men are a little bit stupid and if the prospect of hooking up with the "sister" is dangled in their face then a lot of them will consider it. I'm sorry if I get downvoted for that but it just is a fact that sex makes men stupid (like how other things make women stupid). I do not blame you AT ALL for finding it difficult to maintain a relationship with your sister.
You must never hate the child because the child is just as innocent as you in this scenario. I can bet any money that if the child had a choice he/she wouldn't have chosen to have been born in this situation. Furthermore, the child is your own flesh and blood because that's your niece/nephew. So by all means be mad at the sister but not the child. If the child didn't exist but you still found out about the cheating then I'm assuming you would've dumped your ex anyway. You think he only cheated with the sister but I can bet any money it was with more than one person.
You said so yourself your ex had mental issues. Oftentimes (not all) people with mental issues have damaged prefontal cortexes, which affects their ability to make proper decisions. It was a little bit your fault to expect him to be a reliable long term partner. He may have been a good person deep down, but his mental issues prevented that. Not your fault, but also not your problem anymore. You should celebrate the fact you no longer have to deal with that sh*t.
I know I say not to trust your sister, but I actually feel really really bad for her. She's the one who is stuck with having to take care of a child for the rest of her life, which will affect her chances of getting a husband. She's the one who has to deal with the reminder of the betrayal she caused every day. She was only 18 at the time, so she was obviously very very immature, and now has to pay for it for the rest of her life. Have you never made a dumb decision at 18?
When you start working on your self esteem and self love, then you'll find that naturally you'll be more and more relieved that you get a fresh start on a new relationship, because people who love themselves are physically incapable of tolerating a relationship that is full of toxicity. It's only the ones with low self esteem that stay in toxic relationships. Don't see this as the death of your old relationship, but rather you making space for a new one in your life. Take pity on your sister, and never never take it out on the kid (in fact I think if anything be nice to the kid and send him/her presents on xmas). Forgiveness is more for you than for them. I know what you went through is traumatic, but trust me the vast majority of men are not like your ex husband at all, so have a little bit of faith in yourself that you'll choose the right one this time. It also sounds like you're not so much mourning him but mourning the person you thought he was and thought he could be, so just keep that in mind. You're still young and have a world of opportunity ahead of you. Best of luck!