It sounds as if ex-husband sold the land on a payment plan, i.e. took back a mortgage. Hopefully, the promissory note and mortgage were done properly and give him the ability to foreclose. He sold the land and so these buyers have title. Unless and until a foreclosure is instigated and successful, buyers will still have title and be entitled to occupy the property. Ex-husband needs to see an attorney ASAP, and if the circumstances are appropriate, start foreclosure proceedings. Unless and until he gains title back thru foreclosure, he can't give anything to sister, because he has nothing to give. He could assign her his rights in the mortgage, and let her do the foreclosing and gain title.
To questioners from West Virginia & New York: Although I am licensed to practice in your state (in WV, on inactive status as of 9/13), I practice on a day-to-day basis in Massachusetts. I answer questions in your state in areas of the law in which I practice, and in which I feel comfortable trying to offer you assistance based on my knowledge of specific statutes in your state and/or general principles applicable in all states. It is always best, however, to work with attorneys and court personnel in your own area to deal with specific problems and factual situations.