Just to clarify - I am NOT a solicitor & would recommend she seeks some advice from one.
She should definately find out who the executor is & find out what is happening. Bear in mind, the idea of a "reading of a will in a solicitors mahogony walled office" is pure TV. I'm not sure if the executor has to notify the benifactors until the estate is being distributed, but will be open to others input.
I dare say alot will hinge on the wording of the mothers will & whether it allows the father to over-ride the terms of the will. Also, maybe the siblings could contend that she had exerted undue influence on her parents as she was the only one at home (agian, this would probably depend on the original will).
In relation to "not of sound mind" - I dare say she would need alot of medical evidence to back this up.
It also may be, if she since moved out of the home & was not a primary care giver, that the father felt it was unfair that she should get the whole house. He may have decided the best was to leave the house equally to all of the children (which, without being subject to all the facts in relation to care-giving, I personally think is the fairess way).
Long & the short of it, she needs legal advice!!