I would find a therapist that can help you properly explore your childhood and how it is affecting you today.
I had trouble in my early career because of feelings of not belonging, being afraid to ask for help, feeling powerless. Turns out, it was because I didn't get warm fuzzy attention as a child.
I'm sure you've been trying to come up with a solution regarding your situation with the sibling for a long time, and it must be a very difficult situation with no easy solution, but my opinion is that there has to be some kind of ultimatum so that you can go forward with your life as well.
I'm really careful as I write this as I don't know what your sibling is going through, and I don't mean a single bit of disrespect to your sibling, but is it possible that they won't commit suicide even if you aren't around to help? When faced with the prospect of destitution due to you no longer being available to provide 100% of your sibling's care, do you think there's a chance that your sibling would change and decide to finally take steps to gradually become independent? Since only you know what your sibling is like, you can disregard my comment if it's probable that your sibling will kill self when they're forced with no longer being able to continue their current shut-in lifestyle.
I visited my dad for a week last year, but my sibling apparently stayed inside the entire time.
I don't think being around other people would help. They generally just find ways to hurt me or make things worse in someway, either accidentally or deliberately. All my in-person interactions with people have either been neutral (like asking someone at a store where something is) or negative (every counselor or psychiatrist, other students when I went to school). The only "safe" people are relatives. I think the last time I had a positive in-person interaction with a non-relative was in the 90s. I don't really have anything to offer other people, and want to avoid being some kind of parasite as much as possible.