Legal Question in Business Law in California

You can sue. It will cost you a filing fee, and you'll also probably need to hire an attorney to write the complaint for you, because the issues are not super-simple. The outcome cannot be predicted, except that if you go all the way to trial, a judge or jury will decide your rights and your sister's rights in conformity with the law and the evidence. If there is any way to negotiate a solution, that is preferable to the expense and hassle of bringing a lawsuit. I do not understand your arrangements....why would you pay $27K but not be listed on the title?