If my sister filed a relative petition for me and my visitor visa has expired, what form can she file again so I can legally gain employment?

There is nothing she can file. In fact, you should leave the U.S. as soon as possible because you will not be able to adjust status through your sister with an expired visa. Only spouses, children and parents of the U.S. citizens can do that. Everyone else must be in valid status, or this filing could be for nothing. You have to wait for your immigrant visa outside the U.S., or you must be in some other valid status at all times. I am afraid that you may have already created a problem for yourself if you overstayed your visa and did not change status to some other valid one. Waiting for a green card from a sibling may take over a decade! During this entire time, you must either be in some other valid status here or you must wait outside the U.S. This is the unfortunate reality.