I have to say that I understand where you are coming from. I have never met my husband's 2 brothers or they wives and kids. But, I have met the rest of his family, and I understand why....they are toxic people, and I spend a lot of my time protecting him from them. They live about 5 hours away and his dream is to live by them and I keep saying no. He hates it because my family is by us, but...he moved away from his family before I met him for a reason!
But, if you haven't met them at all, then I think you should do some thinking. I agree that you should have a safe plan for leaving if that is what you decide to do. But, if you want the skinny, call his sister that you have met and ask her. Or, invite his mom to lunch over the phone. Some men are just shy about introducing a woman to his family and need an extra push. Or, you might just find out what you want to know by calling his mom and talking to her.
As for the living together before marriage, well, you have been together quite a while, and it was an ok amount of time if you don't have a problem with living together. (I personally don't, and I think its a good idea, especially before kids).
Ask him what he wants for the future and what his plans are for your relationship. But, be ready in case he says that he doesn't really have any. Then, you can decide what to do. I would probably leave, so have that ready to go.
REMEMBER...HIDE or take away the breakables or important to YOU stuff and the important papers before you confront him. I know this sounds like too much, but it comes from A LOT Of experience on here and in life.
Welcome and good luck, let us know how it turns out please!