Hi Ema, I did feed Ziggy a grain-free dry food. Which is why I was so surprised when he started having issues. I’m probably more in the camp of your mother-in-law, having had a cat with urinary issues who is doing well on wet.
What I sometimes do is buy just one small can or pouch of a new wet food for Ziggy to try. Maybe you could try introducing your girls to a new variety of foods as a snack to see if they come around?
It’s a lot less likely that your cats will develop similar issues since they’re female, but it can still happen. Regardless of whether you try to transition them back to wet, I would just ensure that they have ample access to water and actually drink enough.