All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
First of all, we ask Allah to heal your brother completely without leaving any disease in him, and we advise you to supplicate Allah to cure him, and perform Ruqyah on him as it is beneficial, Allah Willing.
The jurists stated that it is permissible to marry a mad man, let alone someone who is less than him, and that it may be obligatory on his guardian to marry him.
Madness and the like is a disease that gives the person the choice to annul the marriage.
Hence, if your brother is infected with any of this, then it is obligatory to tell the people the truth about his illness, and it is not enough to say that he is sometimes aggressive.
If his parents did not tell the parents of the girl the truth about his case, which they must inform them about, then this is cheating and deception.
It is proven that Abu Hurayrah narrated that the Prophet said: “Whoever cheats us is not one of us.” [Muslim]
Cheating is forbidden and the person who cheats deserves punishment. However, as far as we know, there is no specific punishment about it in the Shari’ah.
Allah knows best.