It's probably especially hard if she was mean to your mom like you say. If she'd been a friend to her in life maybe it would be easier. I'm really sorry you're having to deal with this.

Really sorry, cuz I've been exactly there. My mom died when I was 27, and my step dad was in a relationship 2 months later - and I was cool with that - I already knew he was the type who would need that, would need someone, especially while grieving. I had all sorts of assumptions about the future, that I'd get to know her, even if maybe she wouldn't be forever, and it'd all be OK, and my step dad and I would remain close. Unfortunately, the woman he started seeing was jealous of me, and accused my stepdad of wanting to fuck me, told him it wasn't healthy for him to have me in his life as I wasn't a blood relation. FFS He'd been in my life for 17 years at that point. I had a biologicl dad who was great, but my stepdad was the next best thing. I literally leaned on him at my real dad's funeral. And he was the only one who shared the memories that I had of my mom, the only one I could swap those remember-when stories with. He cut me out of his life, said he had to 'move on' with New Woman. 2 months after my mom died, and he was out of my life as well. I tried to not let it happen at first, tried to fight for my relationship with him. But there's only so much you can do before you end up in the position of being the bad guy. I remember sitting in a parking lot of a bar, after my last terrible conversation with him, where he'd said "I'm going to the bathroom and when I come out I want you to be gone" and I knew there was nothing else I could do unless I wanted to cross over into being the person who's so outta control that the police get called. A few months after that he came and left my mothers ashes/urn on my doorstep, didn't even ring the bell, I opened the door and there she was. I was glad he gave me her ashes, so very glad, because I thought I'd never be able to visit her in that way again (morbid and weird I know, but still) He never did speak to me again. It definitely broke something in me that I'm still now 15 years later not fully recovered from. I threw myself into unhealthy coping mechanisms like you wouldn't believe. Deep down, I held onto a hope that sometime, maybe many years later we would reconcile, and I'd be able to remember my mom with him, and the good times we all shared, again. But sadly it didn't work out that way. A few years later he took his own life, and I still feel such sorrow for him, for the pain he must have felt that led him to that finality, and I wish that he had known I was here all along, waiting and hoping to talk to him again. But maybe that wouldn't have mattered anyway. And I'm sorry this isn't advice, my emotion has gotten away from me here.