Sister Can't Seem to See the Truth

First make sure the kids never forget you are there for them. You don't want them thinking running away to god knows where is a viable option.

Don't trust the kids that she is paying his child support, they hate him they can and probably will lie about him.

Where are the kid's father?

Now the part you won't like, you have to sit back and be prepared to pick up the pieces. If he is a predator he will twist your words around to make her not trust you. Smile, whistle a happy tune and wait.

Sorry I can't be of more help.

A person will not see the truth, unless they want too.

It is sad... that her kids are being affected.
THEY should be her first priority.
As you know.

There is just the direct approach.
People like that do not take hints, very well.
They will be blind to it.

Is her Boyfriend abusive or a jerk?
Even smart women, have boyfriends like that.
There is no rhyme or reason.
She is under his thumb.

An intervention of sorts. Go big, pour all of your love on her, tell her what you need to tell her, and then give it to her and God, because there's nothing else beyond that you can do.

If you think the kids are unsafe you might possibly notify the authorities but lots of repercussions there, so tread lightly...

Once you've given her all the info and help and love, if she chooses not to take it, it's on her, and all you can do is either accept her exactly as she is, or move on.

I have a SIL who is exactly the same way- used to be a very strong woman but now she's with a creep and has done a complete 180. Truth is, she is who she is, not who I want her to be.

Families cause us lots of pain sometimes. Just be there for her and hopefully some day she will see!

You need to get her out of the house, with the kids and go somewhere where they can play while you and your sister talk. Be completely honest and start off with telling her that you love her, and care about her. And then tell her what you told us. That you are concerned for her, and her children, and that you feel that he isnt good for any of them. That he is keeping them away from other people and their families. That isnt healthy, and she probably doesnt see it cause he doesnt let her. Tell her what the children said to you and that if she doesnt do something about it and stands up for herself and the kids then the kids are talking about running away and then the situation is going to get worse. Ask her what it is going to take for her to see what is going on.

All you can do is keep bringing it up and do what you can until she sees that what is happening isnt right, and that her kids are being affected big time. As a mother the last thing you want is to hear that your own kids arent happy and hate being at home. That is so saddening.

Just be honest, and heart felt with her and hope for the best. What do your parents say about this? Or even the Ex husband depending of course the situation with him.

Honestly, you just don't know what is really going on. Kids exaggerate, especially when they don't like someone or a situation. They probably don't like that this man came into their lives so quickly, and then stayed.
If they don't like it so much, why don't they move in with their dad? They are old enough to voice what they want, and it would not be running away.

Worst case scenario, things are bad. Just be there to help pick up the pieces when they fall.

I think there is a good possibility though that nobody is happy with the way things happened so fast, because you care about her, and you are looking for the worst instead of seeing the good. She probably avoids you to avoid being grilled and playing 20 questions! Is the baby happy? Babies can tell you everything by how they act. :)