Re-cap. And make some assumptions
Your grandparent died. That grandparent had two children - your mother and your uncle. Everyone is well over 23 years old so we can assume they are financially independent of everyone.
There were substantial assets in the grandparents estate - money, a house (the family home) and a business. The grandparent left money to their son and the house and business to their daughter. The son had been living in the family home but it was not his - he was not on the deeds. He is unhappy about this and feels he should have been left the business.
What can be done? He can try and upset the will. Will he succeed? That depends on the circumstances. He can at least take legal advice.
Of course, if the daughter who got the house and business also feels that this is all wrong, she could try and re-jig the distribution of the estate. And if she feels that her brother is just being an unjustified whinger, she can simply let the estate be distributed in accordance with her parent's will and let the brother contest (or not contest) the will.
Most of these situations can be forecast in advance - there is generally a history.
mf