Advice please

My SO and I have been together for 7 years. Since I was 19. We've grown to have a mature relationship over time but in November of 2014 he left and immediately started seeing an older sister of one of his friends. This woman is 8 years older then us, still legally married but seperated from her husband, and has 3 kids. After a month of puppy love and messing around she got pregnant. He claims he was devastated and was under the impression she couldn't get pregnant. Him and his family as well as myself feel as if she got pregnant thinking he would stick around. She had found her a selfless kind hearted person and took advantage of him. She had blocked me on his phone and I called at least once a month for 7 months to check on him but was always blocked. Then in June of 2015 I called and it actually rang. As soon as he heard my voice he starred sobbing. He immediately left her and we worked things out. I was even willing to help raise their child. I decorated a room for their child and bought so much stuff for him. Once her due date came around she wouldn't allow the hospital to release her name so none of his family could come see their child, did not put him on the birth certificate, and gave the child her husbands last name. My SO and I got engaged in August and set the date for October (once going through something like that together we both realized we wanted to spend our life together and wanted to move the process quickly). 3 weeks before our wedding she decided to take a temporary protective order against me claiming I would kidnap her child (even though we never even saw him) all based on the fact that I had a room for him. The judge threw that out immediately. Anyways so here we are now, married for 6 months and expecting our first child in September and I still feel terribly afraid that he is going to leave me again. He gives me no reason to think that but the feeling of insecurity I have is overwhelming. Not to mention she STILL has pictures on her Facebook of them together and him with her other 3 kids. We have had no contact with his son because legally we can't and she has made it clear she doesn't want our help which is fine because I see her being the type to call CPS every time the child would return home from our house. But I feel guilt from not having any custody over that child, for feeling like I ripped him away from them and that family, even when he says they are not his family he wants to raise a family with me, I feel extreme insecurity in worrying he will leave again, for her specifically just because for some reason in my eyes she's always been gorgeous (she looks completely opposite of myself so that threw me off when finding out about them) and I feel like she's still holding on to hope that her and my husband will be together with leaving the pictures up on Facebook. I am sure he is not giving her false hope but it's still in the back of my head that he would leave for her again. Any advice please would be greatly appreciated