The story of mothers and fathers not liking the choice their child has made for his or her lover is as long as recorded history. From Shakespeare’s Romeo and Juliet to many current-day movies, the story of prejudice and racial bias in imbedded in the evolution of relationship.
Since you list your age as 24, I’d say it is time for you to be direct with your mother, Let her know that her protests are not going to change how you feel — and that they have more to do with her past than your future. I’d be very clear with her that if she can’t welcome you both, that you aren’t interested in a trip marred by her poor attitude. If your mom can’t accept your decisions, let her know you will wait until she can.
The process of individuation is difficult, but you cannot live your life trying to please your mother. If you do you are likely to hold a resentment that will affect your relationship with her anyway.
I’d also talk to other family members about your concerns to get their input. Right now you and your boyfriend are the target, but they may have an interest in helping you because it may be helping them downstream.