When did she make the promise?
When did she die?
Did she have mental capacity meaning was she still mentally alert?
When did she make the Will?
Please do not shoot the messenger but I do not think you will succeed here.
Your mother was mentally alert and had only made her Will some 3 years or so before she commenced making these promises. As such it is not as if she made a Will put it in a drawer and forgot about it for 20 years.
Whilst she was bed bound she had 2 years to change the provisions in her Will and did not do so. All she would have needed was access to a phone as solicitors will visit to take Will instructions.
I think a judge would conclude that, however well intentioned your mother may have been, had she really wanted to change her Will she would have done so.
If there are any further points please reply I will be happy to response further
I am sorry but what happened in respect of your father's estate was too long ago and I wish I knew the answer to why people are so horrible to each other.