Hi,
I feel simpathy for you.
It is good to open up about your problems, especially problems that happened a long time ago. The thoughts of it may often intrude into your stream of consciousness.
You may also remember that depression spreads and if your sister is depressed someof itmight rub onto you.
When bad negative thoughts intrude, try to think of something pleasant, or better still, try to do something what you ejoy doing. Even if it is something trivial or useless
Hope you get over your problems.