Praise be to Allaah.
Yes, what your brother is going through, whether it is a
physical disease or sihr (witchcraft), or it is a case of mental illness,
or anything else, is indeed a test from Allah, may He be exalted, to His
slave. Every test that a person goes through in this life happens by the
decree of Allah, and it is good that your brother realises and understands
this.
But what is not good and is not correct is for him to think
that he has to wait until Allah, may He be glorified and exalted, lifts this
suffering from him. Belief in the divine will and decree does not mean that
one should not take appropriate measures. Rather when Allah, may He be
exalted, decrees some thing He also decrees the measures that could lead to
it; feeling full happens by the decree of Allah but the hungry person should
eat (to reach the state of feeling full); and having children comes by the
decree of Allah, but the one who wants to have children should get married,
and so on.
It was narrated from Abu Hurayrah (may Allah be pleased with
him) that the Prophet (blessings and peace of Allah be upon him) said:
“Allah has not sent down any disease but He has sent down a cure for it.”
Narrated by al-Bukhaari, 5678.
Thus it becomes clear that as Allah decrees illnesses, He has
decreed remedies for them that are appropriate; those who know them know
them and those who do not know them do not know them.
So what you have to do is to treat your brother kindly
according to his situation, and convince him to seek a remedy for his
sickness. It is good to use both ways together, the usual medical remedy for
the one who is in such a situation and looking into the matter of sihr (witchcraft), which is by his going to a raaqi (one who recites incantations) who can treat him with ruqya by reciting Qur’aan and proper, well known shar‘i (Islamically accepted) ruqyahs (incantations).
We ask Allah to heal your brother for you and to help you in
dealing with him, and to give you the best of rewards.
And Allah knows best.