All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
It is well-known that health insurance, like other kinds of insurance, constitutes a form of risk and aleatory contract and taking people's money wrongly.
So, a Muslim is not allowed to benefit from such an action unless he/she becomes forced to do so due to possible danger to his wealth, honor, or self. Then, one can resort to such an insurance, and the need is estimated according to one's situation.
Here, if your brother or his wife becomes forced to have medical treatment and they find no other means to get it except through health insurance, the danger becomes incontestable; therefore, there is no blame to do so to the extent of the need and situation only.
For more benefit on medical insurance, please refer to Fataawa 408565, 107087, and 82599.
Allah knows best.