I snapped and made my little sister cry.

My little sister (12f) is a very annoying little girl. I (14f) have to constantly correct her mistakes and teach her things she doesn't know. She is very slow and my parents think she has a disability. Anyways, today she was doing her schoolwork and I keep having to tell her to stop and that she is doing something incorrectly. She kept telling me to stop telling her things and go somewhere else and I snapped. I made her cry by telling her she is very dumb and considered disabled and she needs help to do anything and will never be able to be on her own without being babysat and spoiled her whole life. She was considered stillborn when she was born and she has to use an oxygen machine. I am very upset I said those things to her and I told her they were just words and I love her and she is very smart and that it's not her fault for being disabled. She is still crying though and I don't know what to do and I miss my little sister very much.<br><br>Edit: considered stillborn, not was.<br><br>Edit: I've never been in a situation like this before and I don't know how to make her feel better. She still acts very upset and I've tried telling her she is not dumb and I love her and that I am very sorry and I would never treat her that way again and I am considered disabled as well. Please give me advice on how to make her feel better.