Praise be to Allah.
The obligation to adhere to religious duties (including prayer), according to Islamic teachings, is based on whether a person is of sound mind. If your brother is of sound mind, and his disabilities do not affect his mental functioning, then he is accountable. If his mental faculties are affected to the extent that he has no power of discernment, or his discernment is weak, like that of a small child, then in that case he is not accountable and is not obliged to pray.
If your brother is accountable, then his disabilities mean that some of the obligatory parts of the prayer are waived in his case, namely the obligatory parts that he is not able to do. If he is not able to pray standing, then he may pray sitting; if he is not able to recite al-Faatihah correctly and in full, then he should recite it however he is able to, and so on.
For more information, please see the answer to question no. 50058.
It says in al-Mawsoo‘ah al-Fiqhiyyah (10/79):
If the worshipper is able to recite part of al-Faatihah, then the Maalikis, Shaafa‘is and Hanbalis are of the view that he must recite it. The basis for that, according to the Shaafa‘is, is the principle “what is possible cannot be waived because of what is not possible (to do)”, i.e., not being able to do something in full does not mean that the part that one is able to do is waived. According to the Hanbalis, the principle is: Whoever is able to do part of an act of worship must do it even though he is not able to do the entire act, and there is no difference of scholarly opinion concerning that. End quote.
Shaykh Ibn Baaz (may Allah have mercy on him) was asked:
My grandmother can only memorise a little of the Holy Qur’an, as she makes mistakes in Soorat al-Faatihah. Some people in our village told her: if you do not recite Soorat al-Faatihah properly in your prayer, then your prayer is not valid. Is this true?
Her prayer is valid and she is excused, praise be to Allah. Allah, may He be exalted, says (interpretation of the meaning): “So fear Allah as much as you are able” [al-Taghaabun 64:16]. So long as she has tried hard but has not been able (to recite it correctly), her prayer is valid. The Prophet (blessings and peace of Allah be upon him) said to the one who was not able to recite al-Faatihah: “You may say, ‘Subhaan Allah, wa’l-hamdu Lillah, wa laa ilaaha ill-Allah, wa Allahu akbar (Glory be to Allah, praise be to Allah, there is no god but Allah, and Allah is most great).’”
She should try her best if she is not able to recite it, then she should recite “Subhaan Allah, wa’l-hamdu Lillah, wa laa ilaaha ill-Allah, wa Allahu akbar, wa laa hawla wa laa quwwata illah Billah (Glory be to Allah, praise be to Allah, there is no god but Allah, Allah is most great, and there is no power and no strength except with Allah)” instead of reciting Qur’an, and that will be sufficient. As for the one who deliberately omits al-Faatihah, when he is able (to recite it) but he deliberately omits it, his prayer is not valid. But in the case of an old woman, or an old man, who is not able to recite it, then Allah says: (interpretation of the meaning): “So fear Allah as much as you are able” [al-Taghaabun 64:16].
End quote from Fataawa Noor ‘ala ad-Darb (8/236-237).
So strive hard to teach your brother al-Faatihah and how to pray, and be kind in teaching it to him, and let him do whatever he is able to do. If he is unable to pronounce some of the letters, or he mixes them up, there will be no blame on him and his prayer will be valid.
With regard to inheritance, he is entitled to a full share of inheritance. Being sound in mind and body are not conditions of being entitled to inheritance; rather every Muslim is entitled to his share of inheritance, even if he is insane or sick.
And Allah knows best.