Need help with ex-in laws and new pregnancy drama!

I'm at the end of my rope and need some advice. Here's the short version:

My ex husband and I have been together since college and after 8 years of being together and a wonderful 5 year old boy between us, we decided the counseling and fighting wasn't worth it and split while we were still friends. We had been sleeping in different rooms for over a year before my ex moved out and now my mother is living with me so I can take care of her (she's disabled and lost her home). My in laws played a large part in the failure of my marriage. Especially my mother in law, their son could do no wrong and I am to blame for ruining all of their lives including my sons. I've always been nice to them but we have had our altercations over the years. With the impending divorce (it won't be final until summer is over) me and my ex husband are better than ever. We get along great now that we aren't in a romantic relationship. (My husband was a fetichist that was seeking attention outside our relationship during our marriage, and always accused me of cheating with my best friend because he was of the opposite sex.) My mother in law always accused me of it too because she followed me once and I was going with him to a friends birthday party while my husband was at a coworkers wedding. She kept saying I was going on behind my ex husbands back but I always told him where I was and he knew about the birthday party. I'm almost 30 years old and his mother shouldn't be parked outside my house hoping I'll cheat so she can have another reason to lash out at me. So now my husbands been gone for a few months and a month after he finally left on my birthday, I did sleep with my best friend. And now I'm 14 weeks pregnant. It's a very strange situation and I decided to keep this baby regardless of the impeding judgement and shit show that will inevitably happen soon. My ex husband is super supportive and has known since the beginning. He knows I've always wanted another child and didn't want to have anymore with me. My family has basically disowned me except my mother because I'm a whore for getting pregnant before my divorce was final, and today my son finally figured out I have "a baby in my tummy." He sees my in laws several times a week because my ex moved back in with them and I'm worried he will tell them before me and my ex have had a chance to explain to them. They're very nasty and controlling and I'm worried they'll try and turn my son against me and tell them I left their daddy for a new guy when it didn't have anything to do with that regardless of how it might look. 

My mother is also bipolar so depending on the day she's supportive or she thinks I'm a piece of shit and trying to kick my now boyfriend out of my life. She wants me to keep the baby but doesn't want me to have a man in my life (she is a man hater and after my divorce has really clung to that.) my boyfriend is doing his best with all this and is pretty much the only reason I'm still not a complete mess. And my son absolutely loves him. He's known him his whole life because he's been my best friend for several years.

I just needed to tell someone what I'm going through. Any advice is appreciated. Thanks :(