No problem, as long as they physically fit in the space, and you can connect the terminals to it - and are both the same battery type (most probably SLA deep cycle batteries for this purpose)
Electrically, as long as they are the same type of battery type composition (ie SLA, NICAD, L-ION etc) as the one you are replacing, there is no problem.
A 28 AH battery will give you a tiny bit more capacity than a 26 AH, so it will take a tiny amount more time to fully charge, but also will give a little bit more operating time. (A good analogy would be like having a 28 L capacity fuel tank compared to a 26L)
I would recommend looking at the Sonnenschein or YUASA deep cycle SLA batteries from Farnell. Have used them in the past (for other things than wheelchairs) and they work well, and last for many years.
Sonnenschein are double the price and then some of the Panasonics I've got in mind.
Current batteries are going on 12 years, and these don't have to last that long as she will be buying a new chair when she gets around to selling the house and moving into a home.
As long as the newies last more than 2 hours on a full charge, which is what the current ones get, she'll be quite happy.
I would think that two 26AH batteries would last longer than 2 hours, so they must be well and truly gone.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.