Unfortunately, the situation you're in is very difficult. I don't know what the story is of the divorce, but it looks like your husband's ex-wife feels that they are still married, even if by law their marriage has been dissolved. I know that for me that would be the case. If I was to divorce my husband I would probably never re-marry because in my religion and culture we are legally married, but we are also married under God, until death does us part. This doesn't mean anything to many people, which is probably part of today's problems in society.
People seem to see divorce as an easy way out. I'm not saying that people are wrong by getting a divorce, but perhaps marriage means something different for different people.
Perhaps the easiest way to help the girl's is by helping the mom get her life together. If your husband is concerned about their baby-sitter, perhaps he wants to offer help in this area by paying for a better baby sitter or enrolling the girls in after-school programs designed for young girls who are in need of guidance. There's lots of programs out there!
Children of separated parents tend to use parents to get what they want by making them feel guilty etc. and it is hard for parents to discipline and guide children who are confused.
It looks like these girls need attention, (their daddy's attention), girls need to feel that they're daddy's princess so that they do not go trying to get boys attention. They need to feel good about themselves so that they do not try to do things that other girls would not dare do in order to get boys attention.
I'm not an expert, but I have a step-daughter who I used to baby sit since she was a baby. My husband and I knew that it was difficult for her mom to meet her needs and so we stepped in and helped as much as we could. My husband paid for her child care during the week and I baby sat on the weekends because my husband worked on the weekends.
It is not an easy task, but when there are children involved, we need to make their needs a priority and children need to see that they are important to their parents.
Good Luck,
M. Almeida