to the OP, I have taken a while to reply as I have thought a great deal about your experience.
You have siad that as a witness it gave you the courage to speak out despite the enormous pressure you and your sisters were under to remain silent. However have you considered what is the value of a religion that encourages confession but then does nothing to help the victims either legally or emotionally? Their focus is on keeping the reputation of the congregation clean and bringing a wrongdoer to their senses. To handle a matter such as this internally is totally inappropriate and not to warn other parents in the congregation leads to opportunities for reoffending. Your foster dad's feelings were put a head of you and your sisters.
If this had not happened within the confines of a religious group and you did not need a 2 witness rule, it seems to me that your instincts to protect your sisters would have led you to reach out to a teacher or adult who could help much sooner. If you had done this your Foster father would have been prosecuted, you and your sisters would have received counselling and victim support. Your step mother would not have been able to force your sister to accept cuddles from a child molester because not to do so would hurt his feelings.
I understand your fears of going into an institution and of being split up but what you experienced within your foster home was far worse. When we are in a bad situation we fear going into somewhere worse. However there is also the possibility of somewhere better. You have been let down by those who should have protected you whilst your own mother was suffering her own issues. I include the religion in this since your foster parents used their religion to paint your mother as wicked. The religion also failed to look after you not because they are wicked but because their own rules prevent them from offering real assistance to victims since to protect their reputation they have to deny access to legal and psychological services. Elders receive no training in how to deal with sex offenders, counselling, victim support or investigative techniques. They are taught how to determine repentance and the process for dealing with sin.
You have shown tremendous courage through your life but damage has been done and the religion is not blameless.