Hi SammyLi and welcome back here
You are really struggling right now and I am very sorry you are feeling this way. Dealing with these kinds of thoughts and feelings isn't easy at all. You have posted here before and I know you've been through a lot with your sister and mother. The experiences you've had can be quite traumatic.
Are you getting help for your depression? Do you perhaps have a therapist to help you deal with this?
We have a thread here about dealing with trauma that you might also find helpful. Many children of BPD parents find themselves struggling with depression and/or anxiety in their adult lives and many also with (complex) PTSD:
Dealing with trauma: PTSD, C-PTSD and emotional flashbacksYou are in a bad place right now and I am glad that you came back here as you are struggling so. Wanting to help others is certainly honorable. Yet we cannot help anyone if we don't also help ourselves. Now it seems the time has really come for you to put your own well-being first. This does not have to mean that you need to give up on your sister. It just means that you won't completely sacrifice yourself for her and stop to take some time to really take care of yourself.
When you say you give up fighting, do you perhaps feel like you've been fighting the reality of your sister having BPD? Do you feel like you've been able to (start) to truly accept this reality of your sister having BPD and the consequences this has for her life and the relationship you are able to have with her?
Take care