[velma]

I find with a lot of people that they don't like being told off, or to feel like they're being told off as it can make people defensive & not listen so I'd tackle one issue at a time and don't be too preachy about it. Pet shops don't have an obligation to sell the "right" size of anything because there is no right size. And when it comes to wheels there are different sizes because there are different animals. They are exercise wheels not hamster wheels so are there for mice, dwarf hamsters, hamsters & gerbils right up to rats, degus & even hedgehogs. There isn't a set size wheel that a hamster requires but they do need to be able to run with a straight back. I've had hamsters on & off for many years & back when I got my first hamsters there really seemed to be only 1 size. I always thought that's how hamsters ran with their noses in the air not that they only run like that if they spine is curving to fit. Maybe you could find some guidelines & print them out. Maybe along with some Guinea pig stiff so it looks like general research. But maybe leave them lying around for her to find & read as she may take it on board if she comes to it in her own way. You know your sister's personality best. Work out what's made her listen - or what's made her switch off - in the past & use that. Good luck!