What to do when you have 1 incapacitated heir ?

[Sabrina Kane]

I have a deal property under contract with the sister, however she has 4 other siblings of which 1 is incapacitated in a state mental facility he does not have contact with the family. Is there a way or procedure to be able to buy the property without requiring the incapacitated person's signature or waiving the incapacitated person interest in the property. 

The property belonged to the deceased mom. No will. 

So the kids get the house traditionally.

Thanks