I think.. if you are feeling like a ''failure'' it may be because you care about your sister and ... to some extent...her moods. There can be a tightrope to walk.... I wish you well. Keep posting and sharing here, this is a very supportive and caring community

You told your sister that she is not invisible to you. That is HUGE, imo...

I suggest maybe a ''time out'' ..... time can be a healer and can bring ''maturity'' and fresh growth. I can't really give very much advice since I do not know you or your sister, obviously... I do not know if you are consulting a therapist? (sorry if I missed that)

I think that your sister could very likely be bringing out something you Do Not Like about Yourself and that it's affecting your mood.