You should really be seeking legal advise on this situation. A real estate professional could help you put it on the market if all five of you agree to do so.

Matthew,

We are sorry about your delema,,,,,
Your best course of action is to make your feelings known to your family. At the very least, rent should be paid on your share of the property. It's not unreasonable to approach your sister's daughter and make her aware of your need to charge rent....

An unpopular move for your family but possibly essential for you, is consulting a real estate attorney to develop options.

Good luck

You need to speak to a real estate attorney to determine your options of selling your share, "encouraging" a sale or other legal options you may have. A real estate pro (who isn't a real estate attorney) really can't answer your question or give you legal advice.

Good luck.