Don't take this the wrong way: the way you were treated was deplorable. However, I want to note a couple of things.

1) The situation you described is super-duper common. I worked at the unemployment office for years and I can tell you that specific scenario (the boss' wife throwing her weight around) was so common you could set your watch by it. I would go so far as to warn young ladies against working for smaller companies because the situation is so very common and it is easy to get in trouble with someone that you figure doesn't have any power over you until you realize who the REAL boss is.

2) The practice of firing a female employee because she is a threat to a marriage -- so far -- has been found completely legal in the US and is not considered sexual harassment. There was an infamous case involving a dentist, his wife, and a dental assistant who was fired for being "too attractive". The state supreme court upheld the firing, and the EEOC declined to intervene. Basically, if the dentist's wife had wanted to fire ALL of the female dental assistants, that would have been sex discrimination. However, because she wanted to fire one specific assistant because that assistant was "too attractive", that was NOT sex discrimination.

OP can certainly ask her partner to fire this girl, and she'd be well within her rights to ask that (particularly since it sounds like there is some financial investment involved, through her family, which could make her part owner).

With that having been said, even if OP's partner fires this girl, that doesn't address what will happen the next time. OP's partner is who should really be getting the axe, but we all know it's a lot harder to nix someone with whom you have an emotional attachment.