Sorting registration with a new GP while still in hospital (as opposed to trying to sort it on discharge) sounds very sensible.
At the risk of stating the obvious, you say your daughter is using her brothers address to register with a GP. If when discharged she requires visits from Community Services; Nursing, MH, Physio, etc. she might need to double check they have her correct address. Changes of address don’t always update between GP and Community systems as quickly or in the way they should. This shouldn’t happen, but sometimes it does, resulting in Community Staff arriving at the wrong address and the Client not getting the visit they were expecting.