Is the drastic change in my brother's behavior towards his family after his marriage, and his siding with his wife despite proven theft, an effect of magic - especially since he maintains his prayers - and can he be treated with the Qur'an without his knowledge? And how can he be guided to proper conduct towards his family and how can his wife's behavior be corrected?

We ask Allah to guide your brother. Your question includes two matters:

First: Your brother's neglect of his mother. This is an act of disobedience to parents, which is one of the gravest and most sinful major sins. Your brother must fear Allah concerning his mother, treat her kindly, and not accept any mistreatment towards her from his wife. It is also advised to be gentle with the wife, overlook her shortcomings, and cover her faults if she repents. If the wife does not cease her thefts, her matter should be referred to the responsible authorities to seek justice from her. However, it is preferable to resolve these matters within the family.

Second: Can your brother be bewitched while he prays? Yes, if symptoms of magic appear on him. Magic can be treated with ruqyah (definition) Shar'iyyah (Islamic incantations). There is no harm in you performing ruqyah on him yourself or taking him to trustworthy scholars.