Well I think you and your boyfriend should get both your mom and his aunt to clears things up. Don't let other peoples get in the way of your relationship, just fix the problem and continue being with your boyfriend like nothing happened. Good Luck! <3
Your boyfriend is not your aunt. I've heard people talk bad about others and it had no influence whatsoever on how I personally felt about that same person. It may very well be that your bf has his own opinion and is not easily swayed from it by someone else's views.