Most things you've got covered already but would add,
The phone charger cable should be an extra long one. Normal ones are too short and if you're unwell/poor mobility for whatever reason the short ones can be challenging as the plugs are usually in awkward places to access.
Get a pre charged power bank, often phone batteries are the first thing to go in an emergency and having the power bank so you can attach it straight to phone without having to worry if you cant get out of bed to access a plug can be invaluable. You may also encounter jobsworth nurses who refuse to let you charge your phone at the plug so powerbank useful for this also.
Put luggage tags to put on the bag with your relatives name on it, bags often go missing when patients are moved/transferred between wards and have no id in them and can make tracking down the owner challenging. Toiletries bags are also a good thing to have a name in them. Phone chargers always get left behind in the plug when a patient is moved or nicked by other patients so if you can write her name on a sticker and stick to the charger or write her name on in permanent marker.
Dont get big bottles of toiletries, get travel size, family can top up as needed if thats an option (im aware not everyone has family that can do this) You are trying to keep the bag as light as possible.
A non see thru waterproof laundry bag so she can discreetly put clothes that need taken home and washed in. Patients clothes can often be covered in multitude of body fluids and can be embarrassing for them to hand over to relatives.
Particularly because she is not packing the bag herself , a laminated list of what the bag contains and where if it has multiple pockets.
In terms of what bag to get, medium holdall size is usually best. You dont want to be lugging around a giant heavy bag and neither do the staff. When patients get moved, if their belongings dont fit on the bed/patients lap if on wheelchair then they often get left behind to be collected later when someone has time and thats when they often get lost as often no staff available to move luggage around. Paramedics may grab a bag, they may not depending on how serious the situation is and they certainly wont be taking any large bags. Also if you end up taking the bag to hospital for your relative, you dont want to have to heave a large bag around, you may wish to consider one that has the option of wheels as well as handles for carrying
Dunno if its already been mentioned but a handheld battery operated fan. Hospitals are usually boiling.
Earplugs and eyemask, you rarely get a good sleep in hospital due to noise and lights going on and off esp in emergencies.
And yes to the flip flops, hospital bathroom floors are not something you want to be barefoot in.