As the court-appointed administrator, your wife's sister has the authority to hire the real estate professional. Unless the listing agreement she intends to sign is unfair to the Estate, this is probably not a fight worth having, particularly since everyone is agreement to sell the house. At the very least, I would suggest that the sales commission be 5 percent, and that if this person attempts to "double-end" the sale (representing both seller and buyer -- which I do not recommend -- even though it is legal), the commission be reduced to 4%. The Administrator has the duty to enter into contracts that benefit the Estate.
The administrator has the right to choose which realtor to use, as part of his or her duties to administer the estate.
You could go to court and challenge the administrator's decision, but I do not see anything in the facts presented to suggest that a challenge would be successful.
I agree-the administrator's decision.
The answer given does not imply that an attorney-client relationship has been established and your best course of action is to have legal representation in this matter.