How can one deal with an older brother who steals things from the house to sell them and enjoy the money, especially since repeated attempts to advise him have failed?

Theft is a grave sin, but it is not permissible to strike a brother who steals, because the enforcement of punishments (hudud) and discretionary penalties (ta'zir) is the prerogative of the ruler or his deputy. Furthermore, striking could lead to disputes and mischief.

The correct approach is to admonish the brother, remind him of the prohibition and ugliness of theft, and threaten to inform the father or the authorities if he does not desist from stealing, unless a greater evil is feared from such a threat.

One should also supplicate for his guidance.