My daughter was a little younger than 13 when she had difficulty sleeping by herself - she didn't have nightmares, but was terrified of sleeping in her bed by herself.
Our daughter's doctor recommended a counselor - like other here recommended she specialized in working with children. Many health insurance covers this or maybe with a low co-pay so check that out. We met together, then separately. The counselor then set up a few appointments - it worked so well we did not go beyond a couple.
I am not saying that your result will be that quickly - I think it was unusual - I think my daughter just needed someone else aside from family to acknowlege her fear is real, but to also discuss how she overcome this fear.
It is true though - you and your sister would need to feel comfortable with the counselor. If you can get your doctor's recommendation it might be helpful.