There is always JW reasoning that could be used for grandma.
I hate trying to think dub thoughts, but here goes:
1. Jehovah can use any means he wishes to bring about his will.
2. Many times Jehovah has used something unclean to provide what his people have needed. Ex. according to JW's, the magi were atrologers and followed a star from Satan. In spite of their involvement with Satan, Joseph and Mary (who had God's son and his holy spirit) did not refuse them. They let them see the child and accepted the gifts offered to him. Why? No doubt it was for the journey they would be making to Egypt and they were poor and could use these gifts from God's enemies to support themselves on the journey. Your ex and her mother are no doubt then being blessed by Jehovah with unrighteous riches. Those far from God are being used by him to take care of his servants.
3. A prostitute was used to hide the Isrealite spies in Jherico. Would we fault God for having them enter a prostitutes home for protection? Surely entering it would be grounds for disfellowshipping, but God used her and saved her for her good work towards his people. She also came to be an ancestor of Christ. (I think I remembered that correctly!)
I am sorry you are suffering mentally and emotionally as your girls go through this. Thankfully they seem to have wonderful heads on their shoulders and a great work ethic! Keep encouraging them for us!