Sooo... my boyfriend cheated BIG TIME

Me and my boyfriend have been together for about 3 1/2 years and I recently found out that he’s been cheating on me for about a year or more..

Keep in mind we’ve went though 1 miscarriage & we have a 5month year old as well

I cried for over a week and honestly still cry every now and then knowing that I shouldn’t be with him still after all of this but for some reason I can’t get myself to leave

I confronted him and the girl ( which was his ex before me ) and she didn’t care not one bit even though she knew I was pregnant.. on top of all of this they still try to find any and every way to talk , and to make things worse his whole family knew about their hidden relationship and we all live in the same house. Also his mom likes her better than me so she’s constantly rooting for her and says I’m wrong for yelling at her and stuff & she and her daughter are constantly finding ways to keep making her relevant.

This situation is HIGHLY uncomfortable but my boyfriend says he’s not talking to her anymore. I still don’t even really believe that.. they’ve had sex while we were having unprotected sex for years (obviously) but apparently they used protection so I don’t know.

Last Sunday his ex ( the girl ) was over at the house sitting at the table like it was nothing because his mom/sister invited her and had my son too close to her..

I’ve never experienced such pettiness from grown ass women or just adults in general and honestly I just want to pull out the receipts I have on them.. what should I do?