She sounds nice and like she needs a friend. And you seem to like her and she is mother to your DCs' sibling. I don't really understand the problem - what's too close for comfort? You've moved on, she's moved on (whether by choice or not), but you have DCs who will hopefully always have a bond, and that's important for them. In your shoes I think I'd give it a go. If it's the weekend from hell then at least you know you've tried.
do it for your girls , They will appreciate it more than you know , Surely it wouldnt be that long as on ther Sunday she has to travel home and even on the Friday she has to get to yours ,she might not even set off untill the Saturday morning . Try it the once as she has been good to your girls and just that alone it would be a nice thing to do .