My mother has a shallow ceramic sink set into the ground in the garden which holds approximately 10 litres of water at the most. It has nothing in it and she has just been and bought 5 fish to go into it. There isn't enough space for one fish let alone 5. I have told her this before. She hasn't tested the water, it has no filtration, oxygenation etc..
Does anyone have any advice?
Maybe "steal" them when she is not around and return them to the store. Explaining nicely that your Mom has no way to take care of these fish, telling them were she planned on putting them. When she asks where they are, tell her the cats must have gotten them and that is why you don't have fish.
Personally, I would not get her a fish tank unless your willing to do all the maintenance.
After googling I think the fish must have been northern pike, in northern Minnesota.
If your mother doesn't or won't take care of the fish and no one else will, please take the fish back to the store, or give them away to someone who can. Then buy her one of those fake fish in a tank that swims around when you plug it into an electrical outlet. It's fun to watch. My mom used to have one of those. No cleaning needed.