First of all, DO NOT stand up to your live in boyfriend’s family. It is ABSOLUTELY NOT your business to do so. If there is any standing up to them to be done, that’s his business — and the same goes for you with your family. That’s just a rule of thumb.
Second of all, you need to adjust your expectations. I know you figured everyone was going to shower you with love when you moved in together, but it sounds like his family isn’t living up to that. Understand they’re not happy about the two of you together, but that that’s not something you’re going to be able to change. If you stop expecting them to be nice to you, you may have an easier go of things.
Lastly, don’t invite his sister for a weekend again — at least not for a while. If she’s a bad guest then the obvious response is not to invite her back. You are new as a couple and it’s time to create the kind of life you are going to have together. That means that different people in your lives will take on new roles. Your boyfriend’s sister gets to fade into the background if she’s not nice. But don’t make a big deal about it. Be graceful, but be protective and always have good manners.
I hope that helps.