Is it permissible to use a health club membership card in a brother's name to benefit from the services, and does this constitute a violation of Sharia or an infringement on the club, or does it fall under the ruling of theft?

The truth about subscribing to health club memberships is that it is a contract for the lease of a benefit. The lessee is permitted to utilize the benefit himself or through another. However, if the lessor stipulates to the lessee that the benefit must be utilized only by the lessee himself and not by anyone else, then this condition must be fulfilled, based on the Prophet's (peace be upon him) saying: "Muslims are bound by their conditions." This ruling also applies to admission tickets.