He died and left behind a mother, two brothers and two sisters, and the Western court ruled that the mother is the only heir
Question

                A young Muslim man died in a non-Muslim country, in an institution that was taking care of him, because he was mentally ill. He was not married, and he had a mother, two brothers and two sisters, all of whom live in that country. He left a decent amount of money, and the foundation said that the only heir is the mother.

My question is: should they accept that ruling, or is there a shar‘i ruling?
Praise be to Allah.
      It is not permissible to refer for judgement to any law 
      other than the law of Allah, with regard to inheritance or any other issue 
      or situation, even if referring to some other law serves some worldly 
      interest for somebody. Being content with the ruling of Allah, may He be 
      exalted, is one of the conditions of faith, and referring for judgement to 
      His laws is one of the requirements of affirming His oneness and 
      submitting to Him. 

    Please see the answer to questions no. 974 
    and 118682. 

    Secondly: 

    If a Muslim dies and leaves behind a mother, two brothers and 
    two sisters, then the mother gets one sixth, because Allah, may He be 
    exalted, says (interpretation of the meaning): “if 
    the deceased left brothers or (sisters), the mother has a sixth” 
    [an-Nisa’ 4:11]. 

    The remainder goes to his four siblings, both males and 
    females, with each male getting the share of two females, because Allah, may 
    He be exalted, says (interpretation of the meaning): “if there are 
    brothers and sisters, the male will have twice the share of the female” [an-Nisa’ 
    4:176]. 

    What is required is to divide the estate according to the 
    Islamic teachings, and it is not permissible to divide it according to the 
    laws of disbelievers. Whoever issued a ruling that the entire estate should 
    go to the mother and nothing should go to the siblings has ruled according 
    to something other than that which Allah revealed, and has opposed Allah in 
    His rule. So it is not permissible to rely on this false ruling; what you 
    must do is refer for judgement to the laws of Allah. 

    If the non-Muslim laws ruled as mentioned, then what the 
    mother must do is give up to the other heirs the shares allocated to them in 
    sharee‘ah, even if that is done under the guise of buying or selling, or 
    giving a gift, or any other means of transferring the wealth to the 
    legitimate heirs, according to the share allocated to each of them. 

    But if the estate consists of cash money, then the matter is 
    easier, for all the mother has to do is receive it from the organisation 
    that has taken control of it, then give the rest of the heirs their shares 
    of the money, according to the shar‘i division of the estate. 

    And Allah knows best.