Do you know who is nominated as executor in her will? On her demise, all authority is vested in her executor, albeit on a provisional basis until probate has been granted. If she has made it clear that you have next of kin status while she is still living (as with the hospital) and you are also the executor of her will, it would be difficult for anybody to challenge your position.
Disposal of her personal effects and control of access to her property after death are governed by her will, not by verbal instructions given to you while she is still living.
Do you have a real concern that her siblings might want to interfere?