If a brother usurped his brother’s rights, there is nothing wrong with referring the matter to the qaadi (judge), even if that upsets the mother
Question

                My grandfather died 22 years ago and left a lot of wealth. My father only recieved 45% of his share. After my grandfather died my oldest uncle took control of all the wealth. My dad was abroad at the time studying. My oldest uncle never assigned a witness on my dads behalf. And didnt give my dad his full share. My dads question is should be try to get his share back; because he's afraid since my grandmother is old ; she will get hurt from sadness, due to this clash. What should be done?
Praise be to Allah.
    
    There is nothing wrong with your father asking his brother to give him his 
    full rightful share of the estate, because the estate of the deceased is a 
    trust for which the one who takes charge of it is responsible, and he must 
    give it to the heirs listed in shar‘i texts, each according to the shares 
    that have been ordained by Allah, may He be glorified and exalted. If a 
    person seeks his rights, there is no blame on him, either in this world or 
    in the hereafter. Allah, may He be glorified and exalted, says 
    (interpretation of the meaning):

    
    
    “Allah does not like that evil should be uttered in public except by him who 
    has been wronged. And Allah is Ever All-Hearer, All-Knower

    
    
    Whether you (mankind) disclose (by good words of thanks) a good deed (done 
    to you in the form of a favour by someone), or conceal it, or pardon an 
    evil, verily, Allah is Ever Oft-Pardoning, All-Powerful”

    
    
    [an-Nisa’ 4:148-149].

    
    The one who has been wronged should have the right to demand his rights from 
    the one who wronged him, by referring the matter to the qaadi (judge), and 
    to complain to people about the wrongdoing done to him, according to the 
    text of the Holy Qur’an. 

    
    However we should point out a number of important matters to which attention 
    should be paid before referring the matter to the sharee‘ah courts; these 
    include the following: 

    
    Firstly: 

    
    It is essential to certain that wrongdoing has in fact occurred before 
    demanding one’s rights. This requires clear proof, and is not to be based on 
    doubts, conjecture or malicious gossip that is spread by some people, 
    especially if the estate was divided by the sharee‘ah courts. 

    
    Secondly: 

    
    It is essential for there to be a calm discussion between your father and 
    his brother; he should complain to him directly so that each of them can 
    hear the other’s point of view. At the same time, good, knowledgeable and 
    righteous people should also intervene in order to explain matters. You 
    should not give up on finding a solution to the problem before referring the 
    matter to the courts. The courts should be the last resort, after patiently 
    looking for a solution. 

    
    Thirdly: 

    
    There is no sin on your father if his mother gets upset or angry because of 
    his going to the court. Rather she has no right to get upset if the wrong 
    that was done to him is clearly established and he has undeniable proof to 
    that effect. In that case it is essential to convince the mother about this 
    proof and to calmly discuss the matter with her, so that she will accept it 
    and understand the causes that led to that. Then, if she continues to be 
    upset and angry, there is no sin on your father, because his mother has no 
    right to ask her son to give up his rights or to keep quiet about the wrong 
    that has been done to him. 

    
    Ibn Taymiyah (may Allah have mercy on him) said: 

    
    The individual is required to obey his parents in any matter that does not 
    involve sin, even if they are evildoers. This is the apparent meaning of the 
    statement of Ahmad. This has to do with matters that are of benefit to them 
    and do not cause him any harm; if it is difficult for him but will not harm 
    him, it is obligatory, otherwise it is not.

    
    End quote from al-Ikhtiyaaraat al-Fiqhiyyah by al-Ba‘li (114) 

    
    If keeping quiet about his rights will clearly be harmful, then obedience to 
    his mother is not obligatory in this case, and he does not have to give in 
    to his brother and let him off in order to please his mother, even though 
    doing so (pleasing one’s parents) is best in all cases, as Allah, may He be 
    glorified and exalted, says:

    
    
    “Let them pardon and forgive. Do you not love that Allah should forgive you? 
    And Allah is Oft-Forgiving, Most Merciful”

    
    
    [an-Noor 24:22]. 

    
    Whoever seeks high aims, we advise him to disregard what has happened and to 
    pay attention to his mother's likely distress if she sees her sons disputing 
    with one another. At the same time, the one who refers the matter to the 
    courts is not to be blamed. 

    
    And Allah knows best.