Her brother is in love with a divorced woman with whom he had a previous relationship

I advise your father and the father of this woman not to
prevent this marriage, so long as there is no shar’i reason to prevent it.
If your brother loves this woman and she loves him, then it is in their
interests to get married. The Prophet (peace and blessings of Allaah be
upon him) said: “There is nothing better for two people who are in love than
marriage,” i.e., if a man loves a woman and she loves him, then let them get
married, and that will calm them down and protect them from doing anything
haraam. Isn’t their getting married better for him, for her and for their
families than their falling into something that Allaah has forbidden? The
Prophet (peace and blessings of Allaah be upon him) said, addressing
the guardians of women, if a man comes to propose marriage: “If there comes
to you one with whose religious commitment and character you are pleased,
then marry (your daughter) to him; if you do not do that then there will be
fitnah (tribulation) and widespread corruption on earth.” 

But if there is a shar’i reason to prevent that, then our
advice to your brother is to obey his father and to remain chaste, and to
strive hard in doing so. The Prophet (peace and blessings of Allaah be
upon him) said: “Whoever strives to be chaste, Allaah will help him to be
chaste,” i.e., whoever does his utmost to be chaste, Allaah will grant him
chastity and help him. Allaah says (interpretation of the meaning): 

“As for those who strive hard in Us (Our Cause), We will
surely guide them to Our paths (i.e. Allaah’s religion — Islamic
Monotheism)” [al-‘Ankaboot 29:69]

And he should look for
another wife who will help him with his religious commitment and his worldly
interests. 

With regard to witchcraft, if a person regularly reads
Qur’aan and recites the adhkaar that the Prophet (peace and blessings
of Allaah be upon him) used to recite when going to sleep, when waking up,
in the morning and the evening and at other times and in other
circumstances, then he will protect himself thereby from the accursed
Shaytaan. The Prophet (peace and blessings of Allaah be upon him) said:
“Allaah enjoined five things upon Yahya ibn Zakariyya, which he was to
follow and to enjoin upon the Children of Israel…” then he mentioned them,
among which was: “and He commands you to remember Him. The likeness of that
is of a man who is being pursued by his enemy, until he comes to a strong
fortress where he protects himself from them. Similarly a person cannot
protect himself against the Shaytaan with anything better than the
remembrance of Allaah (dhikr).” 

Al-Mi’wadhaat (soorahs seeking protection with Allaah) are
among the greatest things by means of which a person may protect himself
against witchcraft, so recite them in the morning and in the evening, after
every prayer and before going to sleep. 

Make a lot of du’aa’ for your brother, that Allaah may guide
him and grant him wisdom. And Allaah knows best.