My sister betrayed me...?

I'm sorry your sister did that :( what a bummer. I think you're right about court, it would just cause too much drama within the family. It's great you're thinking of the family instead of just the problem between you & your sister. If I were you I would leave the business. If it starts dying, there is a great chance your sister will panic and she might even want you back to save her too! AND if she does, you'll have the ability to say "alright but I'll have my lawyer write the new contract, I wont work for you until it's signed" and she'll be desperate for help, and have no other choice.