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

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.