The BC Schizophrenia Society doesn't have any therapists, but they do have family support coordinators who can help you find the services you or your family need. They also have family support groups for people who care about someone with a brain illness like schizophrenia, bipolar disorder, clinical depression, schizoaffective disorder. That link to a list of therapists I gave you in an earlier post would be a good place to start in looking for a therapist to talk to about all this. Here it is again: http://www.counsellingbc.com/
The counsellors on this site have professional qualifications, so are likely to be helpful. However, you will find that some are going to work better for you than others, which is normal.
If your sister was abused by someone, that doesn't mean that she has borderline personality disorder. Only a qualified professional could say that, and they'd have to meet with your sister to diagnose her.
Childhood abuse can have a lot of effects on a person, from post traumatic stress disorder (PTSD), to increased risk of mental illnesses from the high stress. Fortunately, with help a person can feel a lot better and recover completely.
I think it could be a very good idea to find some individual counselling for yourself to talk over what you remember about the incident with the two boys. Old feelings and memories can be very disorienting. As well, it can help you sort out what you need to do around the other things. Sometimes when one child in a family starts talking about the abuse they remember, it triggers memories in the other siblings of things that happened to them or which they witnessed. Because survivors of abuse often have traumatic amnesia and then come out of it later, sometimes people don't remember some things that happened a long time ago until later in life.
I'm not saying that's the case for you, but if you find yourself having strong feelings or fears around the things your sister is talking about, you might want to make an appointment to talk to a psychologist or social worker about them. Even if siblings did not see or experience any abuse, it is still often very upsetting, and counselling can help. A counsellor can help make a safe space for you to sort things out. It's not something a person should need to go through without support. If you aren't able to afford private counselling, there are some services I can refer you to. I also know of several good books.
You are welcome to share your feelings here.