M.P.
I suggest that you're over thinking this with a focus on what may happen. I suggest you talk over your concerns with your sister and the two of you make a plan that helps you feel more secure about the sleepovers.
How old are your children? Unless they're tweens or older, they aren't interested in your sister's relationship what it is or isn't. I wouldn't be concerned about the couple fighting because your girls will only be with them briefly. Most couples, and you say he is caring, won't fight while kids are with them.
About the possibility he may be a pedophile: that's very much unlikely. Actually, it's absurd. I investigated sex offenses. Less than 1% of men are pedophiles. Less than! Even if he were a part of the possible .5%, pedophiles groom their victims before molesting them. An overnight now and then really doesn't give a pedophile enough time to groom them. Do you trust your sister? Would she allow her boyfriend to hurt them? You do know your sister. She will be there. Why do you need to know her boyfriend better?
I wonder why you're so focused on the possibility of your girls being molested. Such a focus is unhealthy for your girls. I hope you want them to grow up and have healthy relationships with men. Ihope you will want them to live without expecting a.pedophile around the corner. Children rely on parents to keep them safe. I suggest you're giving them the message that they are not safe. That's why you have to work so hard at keeping them safe. The world is so dangerous that you can't even trust your own family. It's good to be remotely aware of sexual issues. It's not helpful for them to be at the front of your mind.
If you can't let go of your fear that your daughters will be molested, I urge you to get help. If you go to church, see if the church has available counseling. If not, find a counselor with whom you can build trust and work through this issue.