This sounds so similar to what happens to me on a regular basis.

I think honesty is the best thing and often has the best results, even though it's also the harder route to take. As someone who's been in your situation multiple times, if you do decide to confront your boyfriend/his family with this, take it very seriously when you address them. Let them know how important this is for you and let them know how this is effecting you in such a negative way. Don't be aggressive or accusatory, tell them you wish things could be different but you are just experiencing this difficulty and would prefer not go. Chances are they have no idea how you feel and are assuming that it would be a delight for you to join them.

I believe that even if it's hard for them, people want to know when their behavior is effecting people negatively, it helps them to consider their behavior in the future and helps them to become more self aware and sensitive to others. I think it also helps to have these kinds of honest, open conversations in the long run, since these people may well be your family one day. Being honest will help them better understand you, and your relationship with them will be better off because of it. That's what I believe anyway... I haven't seen any case studies about it or anything, this is just from my own life experience having dealt with this before. So take it with a grain of salt I guess, you might want to consider that my approach is maladaptive and not right for you.

Be aware that sometimes it has really backfired on me in the past... where I get very negative reactions. Be prepared to deal with that as some people just will not treat you with respect. If that is the case I suggest keeping an open mind and hearing them out, but if they continue to disrespect you, what I would do is just refuse to go and refuse to talk to them about it any further.

I wish you all the best in figuring this out.

And just as a word of caution: consider weighing this against the value of your relationship with your boyfriend as well. As in, if you two end up married or something similar, his family will become your family, and you will have to spend more and more time with them.