If you and your bf have already worked out and agreed to clear boundaries for relatives' visits, then it's up to your bf to enforce them. If he's allowing them to violate those boundaries, then he's breaking his commitment to you and to your relationship. If your relationship has real value to your bf, then he must put you ahead of his family. Otherwise, he's making it clear that you have no future together, and that's a clear signal to you that it's time to move on if you want to retain your self-respect and be with someone who values you enough to keep his commitments to you.
If, on the other hand, you and your bf never established clear boundaries in this area and committed to them, then it's nobody's fault per se -- your problem is a result of poor communication. In that case you can rememdy it by working through it with your bf and agreeing to reasonable boundaries that are acceptable to both of you. That may require sucking it up and allowing this visit for now, but afterwards you should work to make sure that kind of thing never happens again.
In either case it's not your responsibility to enforce these boundaries with your bf's relatives. It's entirely 100% his responsibility. But it is your responsibility to work out those boundaries with your bf and come to an agreement on what you're both willing to live with.
Hope this helps. I can certainly relate to what you're going through.
