Buying a sibling out of the long time family home

[Stacy Lynch]

Years ago I was put on the deed to my mothers home while I was living in it. It was 50% each, tenants in common. My sister came in the picture and she was put on the deed. It was me 50%, mom and sister each 25%. In a family agreement notarized with all 3 parties in the event of death or severe illness I get to buy out my sister at fair market value. She does not get to by me out. My mother now has a severe illness and in a nursing home permanently. Due to that my sister has 50% now on the deed. My sister now wants to keep the house and she has no means to buy me out. My husband and I own a home and can buy her out easily. Would the notarized family agreement hold up in court if she refuses to let me buy her out?