Yes she should, but it sounds to me like you're letting your resentment towards her cloud your feelings about your husbands kids and what's really best for them. Yes she's being a selfish b!tch, but again, your husband spending time with his kids shouldn't be something he feels he has to do on the down-low. And I think he feels that way because of your attitude.
At the end of the day (GOD I hate that expression, don't you?) if neither you nor their dad nor their mom WANT the kids around, where does that leave them? How are they going to feel? Every kid should be made to feel wanted and loved, and I just can't see that happening in this situation.
If all five of our combined children were born to H and I and he and I wanted a night out, we would make arrangements for a sitter.
In the event my step children were with us for a weekend and H and I had something we had/wanted to attend, we also, would arrange to have someone sit with the kids while we attended. We wouldn't go and drop the kids back off with her while we did what we had to do. We respect her alone time, and I feel she needs to respect ours.
The kids know when they will and won't be with us. We've talked to them and asked what THEY felt about the arrangements and the kids are happy.
So I just feel that on the weekends when H's ex has the kids and she feels she needs time away, then SHE should make arrangements as we would.
Whatever the case. We have agreed to having the kids every Friday for the summer, from early morning til after dinner which gives H's ex and her mother more time to do whatever it is they want to do or nothing at all.
Edited to add: ^^ I'm not a completely heartless *****.