Is it permissible for the sister to benefit from the husband's health insurance for his wife instead of herself, by presenting herself to the hospital as the wife, if the wife does not object to that?

Cooperative Takaful insurance is permissible, while commercial insurance is forbidden. If the wife's insurance is cooperative, it is not permissible to present the sister as the wife to benefit from the insurance, because it is impermissible deception and fraud, due to violating the terms of the contract. Allah Almighty says: (O you who have believed, fulfill [all] contracts).

However, if the insurance is commercial and forbidden, it is not permissible to subscribe to it. But whoever subscribed to it out of ignorance or compulsion, may only take back what he contributed, even if it requires a ruse such as presenting the sister as the wife. He must stop doing so once his right has been fulfilled.