Wow....all I can say is "wow". As a parent and a curl girl, their behavior appalls me.
Kudos to you for standing up for yourself. Instead of trying to restrain you to comb your hair, they should embrace you and your hair. It is a part of you. I think it is ignorance on their part as no amount of combing etc will make curly/wavy/kinky hair straight. They need to be educated in a calm and collected
manner about how textured hair should be treated and the differences between textured and straight hair.
I think many of us here have gone through similar issues with non-curly family members. I know I have! So I understand what you are going through. Just stay calm about it. I just wish parents would stop this kind of behavior. By focusing so much negativity about a personal trait instead of trying to find out how to best care for it, all they manage to do is create potentially long term issues regarding hair.