As you are already finding, your relationship isn’t a “little” issue to your mother. You wrote that you and your boyfriend have tried to talk to the older generation. What you didn’t tell me is what their objection is. If it is on the grounds of your being “related,” their concerns are irrational. You and your boyfriend are not related genetically. There is nothing morally or legally wrong with your relationship.
But if they don’t see you two as a good match for other reasons, then it’s only respectful and potentially important to hear them out. They may see a truth or a partial truth that is hard for you to hear but still at least somewhat valid. Your mother may be doing only what mothers do — trying to protect you from making what she sees as a mistake. The question of whether you are “related” may be only a distraction that is covering up more important concerns. At least ask her directly to find out.
If the issue is truly about a mistaken idea about the morality of dating someone who is related only by marriage, then it’s going to be tough to change the older folks’ minds. Issues of strong principle are often not very negotiable. All you can do then is be clear that you love and respect them but that you have to live by your own moral code, not theirs, and hope that they eventually come around to acceptance if not agreement. This is more likely to be successful if you stay out of arguments. There is nothing to argue about. Neither of you are going to change the other. All you can do is lovingly agree to disagree.
My best advice to you is to be more openminded than the older folks are and to not draw such a deep line in the sand that you can’t reconcile at some point. Permanent cutoff from your family would be very painful for both you and your boyfriend. Give your mother and his uncle some time to get used to the situation. If it is clear that you two are truly a committed couple, they may soften over time. Should that happen, you want to be in the frame of mind that you’ll be able to forgive and move on.
I wish you well.