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

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.