Your dad is about to learn what a big mistake this was. I'm sure he was well intended, but most adults would not have said yes. Also, you mentioned 2 nieces? So she has another child with another guy?
Anyway, if your dad has looked into eviction, he's probably already learned that this is a lot harder than most realize. You can't change the locks and toss belongings in the yard. In most states, this will require a court hearing. Also, I don't get why he doesn't want to kick your sister out, as well. This isn't a legal issue, but a parenting one. Her life is a trainwreck, and that's because she hasn't been held accountable for some pretty bad choices. Baby #3 may pop up before this stuff even gets started.
I agree he needs legal counsel. One thing he could do to jumpstart the process is search their room any time they're both gone. It's his home and he has this right. If he finds any meth, call the cops.
The kids are a separate issue. Wait and see how it plays out. But your parents (you didn't say much about your mom) have got to stop enabling your sister.