Does Epilepsy cause you to treat people badly?

My sister insults family members then gets defensive if confronted about it.

Having Epilepsy myself, I can tell you that your sisters problem with insulting family members and then getting defensive is ALL her. It has nothing to do with her having Epilepsy! She must have some issues with the family that she needs to address.

Having epilepsy should not make your sister treat other people badly. Maybe she is worried about her condition and that worry to causing her to be awful to others.
No, you are mean to begin with
epilepsy just means you have seizures, she doesnt need to be being mean to people, thats just lame that shes using her disorder as an excuse to be a jerk.
No , but rudeness does
Only if she feels the world owes her something because she has it and no one else in the family does. Self pity can cause people to be very mean. Tell her to get over herself, that you did not give her epilepsy. I know it sounds mean but in this case she needs a rude awakening.
Epilepsy does not cause one to be rude and insulting to anyone. This is probably the only excuse your sister can think of for her behavior.