Thank you for this question about your boyfriend’s family. This is your boyfriend’s responsibility to manage. He needs to be an advocate for his relationship with you with his family. If he is not doing that, which it doesn’t seem he is, then the work for you as a couple is to find out why he isn’t taking a more direct approach and making some clear requests and boundaries with them. You have stated your age as early thirties and I am imagining him to be roughly in the same age category. A thirty year old man about to be engaged needs to be better able to deal with his family.
Ask him how you can help him do this. It might be in the way of letting him visit with his family alone to have these discussions, it might be in the way of going to couple’s counseling together. In any case the responsibility lies with him to deal with his family and advocate for his relationship with you. If he is unable or unwilling to do this, you may want to have a serious discussion with him about how this will affect your relationship going forward.