What is the ruling on a Muslim man marrying a divorced Christian woman who is older than him and not beautiful? And is it permissible for a Muslim woman to go to fortune-tellers to ascertain if her brother has been deceived by magic?

It is permissible to marry a chaste Kitabiyyah (a woman from the People of the Book); however, if she is not chaste, then it is not lawful to marry her.

Sorcerers possess pacts, incantations, and actions that affect the bewitched, some of which cause separation or endearment, but this only occurs by the permission of Allah.

What your brother is experiencing might be love and infatuation, not sorcery. However, if it is strongly believed to be sorcery, then you must resort to lawful Ruqyah (Islamic incantations).

It is not permissible to go to sorcerers and diviners, as this is one of the major sins, and a severe warning has been mentioned regarding it.