I cant take the mother drama, what can I do?

First thing, you are a grown woman who does not own any apologies. You handled the situation beautifully. You left before you actually got into a confrontation. Your mother is spoiling a fight. She is a spoiled, over-indulgent idiot who has been playing favourites with her children for years. You have your own problems and should be concentrating on them. Do not apologize. If you cannot go there, invite your father and sister to your house for dinner. Do not include your mother or oldest sister. Treat them as if they are not family to you. If your mother asked why, remind her of her behaviour towards you, and demand an apology from her, and state until you get said apology, she is not welcome into your life or home. I am sorry to hear of your illness; you need peace not stress. If cutting your sister and mother out of your life to get that peace, do it! Your father will understand.