Sister signed scammy listing for shared home

Help! I inherited a house in Texas with my sister, who has no business experience and is a trusting soul. Both of us live in other states. Under pressure, she signed a listing agreement with:

--No sales price listed

--8% commission to seller's broker

--1-year listing period.

--I can't find a cancellation clause (although the Realtor said she could cancel while pressuring her to sign).

The contract was then emailed to me, and I refused to sign. After receiving aggressive communications from the Realtor, I have blocked him. Now he is threatening my sister and stating she has obligations to him. I have advised her to cancel in writing.

Can this Realtor enforce this awful agreement against my sister?