Dear Feeling Hopeless,
It sounds like a troubling situation and you are astute to speculate that you might be in over your head.
Thinking about fixing the situation is for a professional. I suggest a family therapist as it is a family problem. Don’t worry about what to say; listening, empathizing and caring is what your nieces need. In addition, exposing them to pleasant distractions like games, movies, nature outings, and music and dance performances, would not only relieve some of the pressure of their forthcoming parents' separation and new home, but would also expand their interests and horizons.
I think it is wonderful for them to stay at your house, maybe participating in cooking/baking/gardening. This is not avoiding the problem, rather just easing the tension. Nowadays, there are so many ways to stay in touch through social media and smartphones... Facetime, Instagram, email, texting, etc. However, a little note via “snail mail” can be savored and treasured too.
Take a look at a book entitled: “The Boys and Girls Book on Divorce.” When your sister and her husband reach the separation stage you can show it to the girls and read it with them. As to what to say...answer their questions briefly and honestly, keeping in mind that their parents are the primary source of information and comfort. Basic statements like: “adults have problems, they try to work them out, but sometimes they can’t; and, it is very hard for kids to understand this until they are older” is a good understandable concept.
Bests of luck,
Natalie Robinson Garfield
TheSenseConnection.wordpress.com
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