The affair doesn't matter. What made him look elsewhere?

Judges usually get mad and punish dissapation of marital assets which you clearly intend. The fraudulent transferred asset will be encumnbered and restored to your name within a year in civil court, at no penalty to your sister, but you will probably pay court costs and it will cost you your otherwise overwhelming female advantage in court.

Give it up. Horsey is a marital asset and created from the joint account. He is entitled to half it's true value becuase you married him and he is breathing, albeit down GF's neck. Perhaps you can claim special equity and some offsets will be made because he's a man. He might get 40%. After 17 years you could trade horsey for alimony. But if you do (what he may even deserve) you will be punished. Welcome to family court.