They make three wheeled bikes. Frequently they are set up as cargo bikes. Also some recumbent bikes have three wheels (sometimes double front, sometimes double back). I looked into a cargo trike when I was bike shopping but ended up with a cruiser with baskets and a trailer. A trike sounds like something that may work for your sister though!
Maybe a folding reverse recumbent (aka tadpole) but they're heavy and might be tricky to get onto a bus especially if she has any sort of physical impairment. Sounds like a scooter or electric skateboard would probably be off the table too.
Regular trikes have the same issue of extra weight/bulk and are extra dangerous cornering because of their tendency to tip over. How far does she need to go exactly? Is eliminating the need for the bus entirely an option?
Otherwise, maybe something like this https://www.walmart.com/ip/TopMate-ES32-Electric-Tricycle-Foldable-E-Scooter-Seat-Adults-15-mph-Lightweight-3-Wheel-Mobility-Scooter-Reverse-Function-10-Tires-Trike-Commuting/746353277 but understand it's not going to be something she can or should ride very fast, especially when cornering. Not sure what the general name for this style of bike is, but it might be light enough that she can carry it while still being a trike.
I think lectric and rad power bikes both make an electric trike. Probably not able to be taken on a bus though.
Their trikes are usually heavy, even if you could fit them on a rack his sister might have a tough time lifting them onto the rack. IIRC the Lectric trike is like 70lbs base weight.
Maybe a self balancing 3 wheel electric scooter?
Ebikes, scooters, longboards: Whatever floats your goat, this is all things micromobility!
"Transportation using lightweight vehicles such as bicycles or scooters, especially electric ones that may be borrowed as part of a self-service rental program in which people rent vehicles for short-term use within a town or city.
micromobility is seen as a potential solution to moving people more efficiently around cities"
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