The person who can sign for an estate is the personal representative, and that power comes from a court document called letters testamentary (when a will names them) or letters of administration (when there's no will, or the named person can't serve). Strictly, an executor is someone named in a will and an administrator is someone the court appoints without one. Courts in most states use personal representative as the umbrella term, and heirs and investors alike say "executor" loosely for all of it.
Until those letters are issued, nobody can sign for the estate. Neither the sister alone nor all three siblings together can bind it. The exception is property that never went through probate, like a house held in joint tenancy with a survivor or held in a living trust, where the survivor or the trustee signs and there's no case at all.
You can put a contract in front of an heir before appointment, and people do, but the estate isn't bound until the appointed representative signs or ratifies it. Several states also require court approval or a notice period before an estate sale is final, so ask the estate's attorney which track this case is on.
Ask her for the case number and the attorney's name, then pull the docket yourself. Probate filings are public in most counties, though how much shows up online varies by state and county.
The part that surprises people is the calendar. Even a clean case has a creditor claim window, often several months, and many title companies won't insure until the representative is appointed. Write a long closing window and a right to extend.
