Legal Question in Business Law in California

There really is no way. You both have to sign it and then someone has to cash it. All you really can do is make copies of both sides of the check before and after signing it and then sue her if she doesn't give you your portion, or go down to the bank together and take your share in cash when the check is deposited.

Too bad you can't trust your sister. Without knowing the amount involved or how far apart you live, I can only suggest, as Mr. McCormick did, a joint trip to the bank to endorse and cash or deposit the sums, or perhaps having her give you a (postdated?) check for your share, or at least some written acknowledgment of your share, so that if you have to go to court, you have some proof. Another possibility might be to have the maker of the check replace it with two separate checks, although this might not be easy.