@sid333,
It would be nice if your sister could use this circumstance to teach her children that jealousy is a wasted energy and only makes them resentful and sad. There are always people who'll have more money, more toys, more everything - the key is to teach children to focus on what THEY have and be content with it.
They have had the opportunity to stay with you and enjoy many perks that came with it. They blew it and it would be a good lesson to teach them why exactly.
Gratitude can teach them to be empathetic and most importantly be more thankful and happy.
