Unless the 16 yo is being betrothed to a king several hundred years ago, it isn’t ok. It wasn’t ok then either, but at least there was diplomatic strategy to “excuse” it. Make sure her family is inheriting a lot of land, wealth, and serfs. If not, there’s no way to explain it.

I’m truly sorry you’re in this situation. Call authorities. Send your father to prison. Or leak the info to someone here to do it for you. Do her parents know?