Estate Law Questions? Ask an Estate Lawyer.
Your recourse would be to file a lawsuit for "partition". This is a suit where a judge orders a property sold so that an owner can "cash out". It is an absolute right and can not be denied.
Sister would be able to add in any mortgage payments/taxes/insurance that she had paid into her portion of any proceeds, but you could also argue that since she was receiving the benefit of living rent free, the court should offset this amount by a reasonable rental rate of $250/month.
So you would need to get a real estate attorney to file the partition suit to get a judge to order the house sold.
If I have answered your question, PLEASE CLICK THE GREEN "ACCEPT" button NOW, so that I receive credit for my work. Bonuses are optional and greatly appreciated. If you need additional clarification on this question, just click "Reply" and I will be happy to help you. Please give me as much detail as you can so that I can respond after receiving all the relevant details.