I just thought of another item that may be a problem for your sister. Normally, when someone deeds land via a warranty deed, there is an exchange of MONEY and/or "other good and valuable consideration". In the constitution for the united States, it says ONLY contracts involving twenty-one silver dollars or more are contestable in a court of law.

In other words, did sis PAY your mother fair market value for the land or was it a GIFT? Ask the lawyer if federal gift tax is in play here. If sis is looking at owing uncle sugar a large amount of money in the form of gift taxes, she might be more willing to negotiate.

This about sums it up:

https://www.irs.gov/businesses/small-businesses-self-employed/gift-tax