That's quite a situation. Any time you find yourself in a position where it seems like lying might be in your best interest, it's not. It will ALWAYS go sideways eventually. It's like reality is designed to uproot lies over time. I'm really really sorry you had to learn that in such a severe way. That's really sad for you and your dad and your sister.
You're right. It is a bit baffling to determine who to talk to when supposedly the police and social services are both fully informed. The first thing that comes to mind is a good pastor, but part of the reason I would think that is because the one that I'm most familiar with worked in department of children's services for many years, so he was well informed in areas like that.
My second thought is having a lawyer who specializes in social disputes and defamation, but the tricky part is finding them and getting them to take the case, because they'd have to be confident that they could win and receive the fees for their time so they can pay their own bills.
True or False: the main issue is that your dad's ex now has full custody of your sister and your dad's custody was wrongly stripped due to the lying, and your dad's ex is a pathological liar who is perfectly fine not letting you or your dad see your sister..? And so the goal would be to actually have the ex's custody stripped because of her false-testimony, defamation, etc. and have her cover the legal fees incurred.
I mean.. best of both worlds would be a paster who used to be a lawyer or a lawyer who used to be a pastor, or a lawyer who used to work for DCS or something like that. But, it's really seeming like a good vicious lawyer is what's needed to knock down doors and demand this be straightened out. There are no easy solutions now.