You've got a type of "groceriebag" they sell for strollers. The ones that look a bit like
They're not expensive (about $8,- overhere, don't know about the US), and sell in babyshops etc. They're great to put on a wc that has pushbars. You simply attach the sides to the handlebars, et voilà!
Others get creative with their sowingmachine. They make a little flap/pocket (very simple, just folding fabric dubble, sowing the sides shut and leave only one side open or put a button or zipper there) and attach that underneath the seat. You can do this by using velcro. Above all that's a great place to hide important stuff like a money, creditcard etc. No way somebody can steal it from there without being very very obvious.
You can also use a normall bag. A rugsack could work if you make the straps as short as possible and put the left one over the left side of the back of the chair and the right on the right. A bag with only one strap could also work depending on the chair and user, by putting the strap cross ways over the backrest, leaning into it a bit (the person sitting) to secure it. This can only be done if the strap isn't long and the chair isn't low with a very short back.
Important for you if you decide to use a normall bag; watch out that the bag doesn't hang too low or is to big. Pushing a wc and keep bumping into a bag is so irritating.
You could also think about fanny packs. One for you, one for the person using the chair. If you rent a chair; it might be that they're very wide in relation to the person using it. Putting a small bag next to you while sitting, could then also be an option.
Small tips for using a wc;
- take an old rag with you for those rainy/wet moments. If your sister wants to move the chair around a bit herself, nothing is more irritating and possibly dangerous than wet rims etc. Also that'll be asking for it to get dirty hands. For you pushing it can also come in handy to dry parts of.
- if you take an umbrella; sis and you better watch out! It wouldn't be the first umbrella-ouch-my-eye-accident due to the height difference between the two of you and being new to it.
- look at the fabric of the chair when it arrives. If it is anything like skai, that'll stick very uncomfortably to the skin in warm weather and wearing something like shorts. Putting a simple towel over it will keep that away and it'll sweat a lot less.
- same goes for the handlebars. Some rentalchairs have got yucky fabric on those. (don't ask me why!) Again, a simple piece of fabric will do the trick if needed to give your hands the best time possible.
Have fun!