which would never have happened if it wasn't for the increased time they suddenly started spending there, and their eventual moving in.
See I disagree with this. The split wasn't down to the dsc moving in. It was down the father and his parenting and his allowing that behaviour to happen. Who let's their children treat anyone that way. Chances are, that if he was a better partner and parent, it could have worked.
Assuming, everything you said is accurate and the behaviour was just poor on the teens sides, he let it continue and placed the blame on her.
I can totally see how when more kids move in the dynamic changes and couples end up splitting. Even when people are all doing their best.
But knowing it would split you up, is the bit I struggle with.
When dd decided she could no longer see her dad and went from spending 3 nights a week with her dad to non, it was a very upsetting time for her. It was very emotional. My job as a parent was to be there, help her through and support her.
I needed Dp to support us as well. If that happens with ds, I will do the same for him. What I don't need is the added pressure of knowing this would likely end my relationship too. I don't need that niggle in my mind. I don't need an extra thing to worry about. Which is why, if dp felt this way, I would just rather know, so I could choose wether that's a relationship I would continue. And it's not.
I do think alot of the time, where step parents are involved and there's problems. It's usually down to the adults involved.
And I am not saying the step parents fault, though sometimes it is. In the situations I know in rl and the ones you read here, it's usually because the actual parent of the child is a bit (or alot) shit at parenting.
I know loads of blended families and have done through my life and there's only one situation that stands outs as the step mother being a dick. The rest were caused by dad's who didn't parent well and expected the SM to pick up the slack and take the shit that occurred due to the poor parenting.