I feel for you. You try to help someone out and they stomp all over you. You seem to have trouble keeping firm boundaries, i.e. when they moved your bedroom suite! THAT was the time to get them out....actually the boyfriend should NEVER have been there. You set yourself up for that one..He's not pregnant so he can leave immediately...especially if he never paid anything. Whenever you have the exchange of money, it can get sticky, but if you have proof the rent has not been met, that would be in your favor. I have also heard something about the difficulty of eviction, but it can be done. Landlords have to do it all the time to the ilk of your sister and her BOYfriend. If you are out of state, that is a definite handicap. Where are your parents in all this, since they seem to know? Do they favor the sister and support all this junk? Can the sister move in with them? (just a thought, although given she has a WHOLE HOUSE to do with as she pleases....will you soon see nursery wallpaper on you bedroom walls?????..) This has gone on so long, she is rather entrenched, and obviously used to getting her way. You will have to "steel" yourself for the battle, if you take this on, as I am sure you will be challenging some long standing family dynamics....this is about more than just your house, if "yo know what I mean.."? Define what you want and have a way of enforcing it, and don't let your emotions pull you into the drama your sister is SURE to play.
OK, I'm editing this after reading some others' comments. I agree about the idea of calling the police. If there is no written contract, and they have been there on your good graces, and especially if they have NOT given you money (even if they did, you could give it back, since it is not enough anyway) that may be one of your easiest and cheapest options. You could go to the police station in your district and talk to them first, explaining the situation and see if they can help you. It is a possibility worth pursuing, but get ready for the backlash as your sister screams "victim", especially if she ends up being escorted out by the police..(OMG, my sister called the COPS on me..!) Then you must get their stuff out in such a way that is "legal" (It may just mean putting it out on the curb...ask the police) TAKE VIDEOS/PHOTOS of everything so they can't accuse you of stealing from them. Depending where your parents stand, perhaps it can be taken to their house (preferably by them, once it is out of your house) How much do they want to help....BOTH of you? I would say it is best to keep them OUT of the house once they are physically removed, so you may need to get the stuff to the curb yourself.
I wish I could imagine this magically going well for you and your sister mysteriously wanting to "do right" by you all of a sudden, but you will probably have a big fight on your hands, so don't let that discourage you from doing what needs to be done...and don't let it get you down. It would be in your favor to have some REAL renter-prospects waiting in line to actually rent your house properly to add clout to your actions. Otherwise the argument by your sister will be the house will just be empty anyway and WHY can't she and BF live happily there at your expense? Nonetheless, it is YOUR choice to leave it empty, if you choose, which I have done with my house for months at a time. Don't be BULLIED anymore!
Another thought is if you have any hopes of being the happy aunt to this new little one on the way. Your sister will use any of these feelings against you to punish you and keep you from that. Perhaps that could be diminished if you gave a nice baby shower, but that may not be possible for you, depending how this plays out. Just be prepared as much as possible for the "fallout".