I, also, am a parent of teens...Sometimes we might say something and not realize how it makes our kids feel...Perhaps you need to sit down and tell them how it makes you feel...I also feel like a burden and feel like I need to apologize for the cost of money or time.
Like Saysusie and pearl have said...you did not cause this, nothing you did made this happen...so there is nothing that should cause you to feel guilty. Talk to your parents and have them talk to the other siblings. I like the idea that Rob said. Perhaps you should talk to the doctor or nurse and have them tell your parents what it is doing to you and that their "jokes" are not helping.
I have to weigh everything that I say, because sometimes it can be misconstrued to mean something I didn't mean, but that is how it has been taken...it is a good chance your parents don't realize just how this makes you feel. I know you have said something in the past, but they might have forgotten.
Take care, honey,
I have promises to keep and miles to go before I sleep, and miles to go before I sleep.......Robert Frost