بسم الله الرحمن الرحيم
(Fatwa: 140/16/Tatimmah/M=03/1440)
In the question mentioned above if Ibrahim (your cousin) has insulted religion and the angels and thereafter read Kalimah Shahadah sincerely i.e. he became Muslim wholeheartedly after being apostatized then now he is a Muslim. And when he has repented from his sins then his sins are forgiven. The sin of theft is also forgiven by taubah but it is mandatory for him to return the stolen goods to its owner.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband
