abitmessedup, I know that you have had lots of advice here, much of which, you have said yourself isn't what you want to hear.
Why is that? It's almost like you are just going along with this so your mum & SD don't get upset. Which is wrong, you are the one who shouldn't be upset & in this impossible place. If it were me, as my daughter you wouldn't even HAVE to make this choice, as that man, husband or not would be out of both our lives for what he did.
Your mum is saying that she accepts, maybe even condones what he did to you, I know you say that isn't so, but she is...why else would she stay with him & ask you to cary on as if nothing had happened? And now, because she can accept it, you feel as if you should accept it & you are passing that request of acceptance down to your son. Which just keeps this man feeling smug that he has got away with ruining your life. Because he has. And yes, I am afraid that that does give him cart blance to think he can get away with it again.
As others have said, they were abused in the same room as others, in a house full of others...it is possible that if this person WANTS to abuse your son, he will find a way & what is to stop him this time? No consequences for what he did before. The mother of the child he abused still loves him & stays with him, you still see him...what consequence there?
I am very passionate about this, as my brother abused his daughters (2) & his daughter's friend. He did his time, but shows NO remorse, thinks he was right to do what he did, as he 'owned his family' & the friend willingly came into his house, so she was 'up for it' (his words) too. He is out now & has a new name & is in a new county. We are not allowed to know who he is or where he is. He is almost free to do what he likes.
While his daughters give themselves to anyone who offers them attention, because they think that is what love is. They are out there somewhere, no one knows. They ran away after their mum vowed to stick by him (even tho she has a younger daughter at home who was around 1 when he went inside, so only 10/11 now), because she has never known life without him. But as it turns out, he isn't allowed near them. But the doesn't mean that the wife isn't where he is, right now, playing happy families. We know that he called her after he came out, which he shouldn't have done.
I loved him so much, he was the eldest & he was my hero growing up. I would sleep in is bed when he was away at friends just because I wanted to be close to him. Now I hate him because he made all that so tainted, it all felt so wrong.
Even if he were still in the 'family' there is NO way my dcs would EVER see him. In any way. That would be like I too was condoning what he had done.
My other brothers said that "the day I had to ask my own dcs if their Uncle L had touched was the worse day of my life & I wish they were never in that situation in the first place"
One day (if you continue to see this man & allow your son to) you will feel compelled to ask your son if your stepdad has touched him, because the wondering, even if you are there 100%, will also always be there too. How will you feel, having to ask him that? How will you feel if he says 'Yes'?
Only difference, abit, is that my other brothers didn't know about L's abuse.