My sister has trouble with a mouse and I suggested a trackball to her instead - something like the Logitech MX ERGO - has anyone any experience with or any alternative suggestions on what might be good?
If it is of any use, I have two suggestions you may want to consider. Before I make them, I should tell you in advance that I tend to favour older* peripherals. Meaning: I do not know what kind of luck you will have getting them to work with your sister's particular operating system.
With that said, these two particular trackballs leap to mind because they do not require fine motor control and should be easier to use for arthritis sufferers. Suggestion #1 is the Kensington Expert 4-button USB Trackball:
(Photo credit: not me.)
The Expert combines a large trackball with 4 big, can't-miss buttons. However, the milled black ring you see around the trackball is what it uses for scrolling. You
may be able to re-map one of the 4 buttons for scrolling, but I am not certain of this, and I do not want to tell you you can when it turns out you cannot.
Any road, the model shown in the picture is an older model of the Expert and Kensington offers other, newer models. A search for Kensington Expert on That Internet Auction Site should turn up a variety of different models with the same basic design and you can pick and choose what you think will work best.
Suggestion #2 is the BigTrack Trackball:
https://www.bltt.org/hardware/geniusroller.htmSame basic idea, but this trackball has only two buttons rather than the four found on the Kensington. However, I mention it because it is a fairly bomb-proof trackball originally designed for children that has a much wider base and is specifically meant for use by people who do not have good fine motor control. If this one is of interest, be sure to confirm with the seller before you take the plunge that the one you are being offered is a USB model because it was originally offered with a PS/2 connector before it was updated with a USB one.
As with suggestion #1, you can find the BigTrack on eBay at prices ranging from the reasonable to the ridiculous.
I do not know if you will find either/both of these two suggestions to be of use, but they may at least help to steer you in the right direction. In the meantime, can you please post back and tell us what operating system your sister is using? That way, someone can comment on the availability of drivers, third-party software, etc. that might enable you to utilise one/both of these options for your sister's particular OS.
*I have found that older peripherals tend to be built with greater care than their present-day counterparts, so I am willing to trade a bit of work to get them to work with newer machines in return for longer service life.