Today, I reread your post a couple of times and realize I misunderstood the relationship part. It's the heirs of the brother who lived in the house who died that is challenging the will. ie, there is only one brother. I see the dilemma more clearly.
It seems to me that the brother's heirs have no legal claim on sharing proceeds from sale of house. They can file in court making that claim and it will be up to the court to decide. Is that what his heirs have done?
Court action is expensive. I would compare the cost of going to court with possibility that his heirs could win with the cost of giving his heirs 1/3 of the proceeds of sale of house. I would make a decision based on the likelihood that heirs would win and how much each sister would get in that case as compared to how much each would get if the house sale is only divided 2 ways. I would rely on your mother's attorney's estimate on both.
Sounds complicated to go this way. However, her attorney can simplify this using $ numbers, if he thinks it's possible the heirs will win. When a will is challenged the cost of the attorney hired by the executor is paid from the estate before funds are distributed. Ask your mother's attorney, actually he is the estate's attorney, about paying court costs.
However, the decision to fight the heir's claims involve more than the black and white of money. Emotions influence decisions too. What is important is to recognize them and decide what is most important to the two sisters. What is their goal and how much are they willing to risk to obtain their goal.
I suggest to consider the possibility of reaching a compromise. I would consider mediation. I think the cost would be taken from the estate before distribution.
I would get organized by listing my goal and compromises I might be willing to make. List questions to discuss with attorney about what would help you reach that goal.
I would need to make a sort of flow chart that is based only on $ amounts. Your mother's attorney can help with that. I would also write down an organized list of emotional issues. I would get help from someone who can be neutral to help me know how to make and reach goals. The attorney may be able to also help with finding a counselor to do that. I have to put thoughts in writing because otherwise I go in circles.
For me, having peace and love now would be more important than following my mother's wishes as set out in her will. It's been 10 years and circumstances have changed. It's quite possible, if her mother knew current circumstances she would make a different decision now. My mother would want peace in the family.
My first resonse below may not be helpful.
*****************************
Thoughts about what? If the will has already gone through probate hasn't a judge closed probate? It's been 10 years. On what grounds can your uncle contest the will now.
Your mother has an attorney. Why and what are you researching?
Those are my thoughts. However, I really don't understand the situation you briefly described. I consult with an attorney on legal issues. If I believe he will represent me,.I hire him. When he tells me I can save money by doing some research, I try to do it. But only with specific direction from the lawyer or his office.