I'd say keep the Yaris for driving your mother around, and the FR-S should be able to handle most of the other everyday uses. Sounds like you have at least 2 cars so you can afford to keep more than one? May not be what you want to hear but you should probably sell the Miata if you can only keep 2. Miata and FR-S would serve the same general purpose, small fun to drive 2 seat car. The only thing you lose is the convertible top.
p.s. I know the FR-S has 4 seats, but in my mind (and probably in most of yours) it's going to be used as a 2-seater with room for storage in the back.