Hi there. I'm sorry about your ex, father to your kids. That's so hard on them to lose a parent! Even a distant parent. I can understand your feelings and how this is awkward. No, you have zero obligation to send them to visit her. But how do they feel about it? What do you think her motive is? Is she trying to be loving to them? Having someone love our kids is never a bad thing. Could she do that by just coming to an event to watch like if they are in a play or play a sport or in the school band? Can they write to one another rather than having over nights? I think you should check in with your girls to see casually how they feel about it. But no, you do not have to do anything with the wife of your ex after he's deceased. Zero obligation to her.
People do funny things right after someone dies, you'd never recognize it as grieving. If your ex's wife loved your ex (although I will say from the way you described him that he doesn't sound that lovable) it's possible that she feels his daughters are the last living connection she has to him. Also, you never know what went on behind the scenes. Especially since she was the one who communicated with you all along about the girls' visits and child support, she might have been the main reason your ex saw the girls at all. She might just be a loving person (evidence: she put up with your ex) and if your husband wasn't being something of a foot-dragger on the issue, would have had a more complete stepmom relationship with the girls all along.
In your shoes, I'd ask the girls. They're old enough to know if they like her or not and how they would feel to visit with her. If they would be glad to see her, set up something like all of them meeting at a fun restaurant for lunch or going to a movie, nothing as lengthy as spending the night. (Unless, of course, they say they would really, really like it.)
You might feel irritated because you feel like she's taking credit for the children you raised, but maybe she's just trying to be a stepmother. Probably once your ex died, she is not legally their stepmom any more (likely the term would be something like former stepmother), but my stepmother is still my stepmother in my eyes even thought my dad has died. She acts the same way she always did, comes to family parties (even ones where my mother is -- they get along), and she even paid for my nephew's last year of college (after my dad died). Your girls are getting older and have their own relationships with people, and have the right to see their friends and people who care about them, unless you think the person is actually dangerous to them. I realize this makes you grit your teeth, but they will soon enough be able to see her (if they want) whether you have an opinion about it or not. It might help if you get yourself in a mindset of thinking of her a slightly overbearing aunt, and making your judgements accordingly. :)
Could she be lonesome after losing a husband who perhaps took a lot of care. It sounds like the new house has more room for company than the old. Anyhow, you have no obligation to feed her need. How do the kids feel about this. How would you handle the distance?