Hmmm. Tricky.
I have an almost 11YO step son who behaves/talks like a stroppy 14 year old the majority of the time, and I have my own kids: a 5 and a 2 year old girls. We have him every other weekend, Fri-Sun. If my DH had to work that weekend, I wouldn't bat an eyelid at "babysitting". I would happily do it. Even if DH only got a few hours in the evening with his son, it would be better than him getting none because I won't have him.
DH does school runs on occasion for me, looks after them if I have a lot of errands to run, he will do bedtime if I'm poorly etc. When you marry someone who was child/ren with an ex, you have to accept that and be prepared to look after them like they are your own children.
My ex is currently dating a 19 year old (hes 27) and that is causing some friction due to her wanting to go out on the lash all the time and him caving in to her teenage demands. I would much rather my ex be with a woman who is willing to love our girls the way my DH loves them.
My Dads 2nd wife was a fucking horror bag nightmare, who despite having 3 kids and grandkids of her own, hated the fact that my Dad had 3 kids from his first marriage. My Dads 3rd wife is amazing, they've been together ten years and she is like a mother to me, and that has included arguments, fighting, cuddles, support.
If your DDs step mum is having her at the weekends when your ex isnt there, Id be grateful, not moaning. Teenagers are hard, even more so when they arent yours and you cant discipline them.
Theyre just my thoughts on the subject.