I would not do anymore sleepovers with the cousins. Meet them instead at a park or the mall or mini golf or a movie==each family paying their own way of course.
If the subject of a sleepover is brought up, say something along the lines of "There won't be any more sleepovers due to what happened at the last one." Repeat, repeat, repeat. No explanations, excuses, promises, etc.
I would also cancel the vacation with this family. In the other thread you said your kids don't want to go back to dw, so that is the perfect out. Give exsil a list of websites if she wants to plan her own vacation. I would also be leery of taking the cousins with me on an extended vacation. Do stuff around your home location with them instead if you feel the need to treat them.