Mother in law left home to 6 children in her will when she passed. 2 sisters living there at time of her death had right to live there for 2 years paying their bills, land taxes and upkeep. The land taxes have not been paid for 2008 and 2009. Home is up for sale, but with this economy. Is there anyway to get sister in law out of the home, since she is not paying land taxes and living rent free for almost 4 years now?

Go back to the probate Court (or whatever Court handled the estate in your area) and inform the Court that the tenants are not meeting their obligations.

The Court will issue an order that they either conform to the terms OR issue an Judgment against them (following a hearing) for the arrearages.

I trust you just found out about the unpaid bills and didn't allow them to stay for a period after you found out? If you did nothing it could be construed as you approving of the arrangement.