'even if it is from stranger' if you were going to any support group or counselor chances are they would be strangers too, and some times that is a good thing, you can be very honest and the strangers are not emotionally involved in your situation. A good example is your mother, she is in 'proctect my child' mode and is not thinking logically, but emotionally. This also is probably an embarrassment for her.
(((Blessings to you and your family))) for this nightmare you are going through.
You must get help for the children, your sister is only thinking of herself and the drugs.
One has already been molested, how sad. Get them out of there immediately! and stay on the forum and read all of the info you can about addicts, if you can not find an Al Anon group in your area they are online too.
You are a strong person and the kiddies are so fortunate to have you!