@packnerz Ehh, heels aren't practical for
anyone. Practicality has its place and I don't recommend anyone spend much time wearing heels. But you spend all your time being practical and acknowledging your limits you'll end up internalizing the idea that you're not a complete person, which is probably worse for you than the risk of tripping. Nee-san worries about her so much it's a bit stifling; she's gotta let her live a little.
Yukiko'll just have to get him to take responsibility by, say, putting his arm around her to make sure she doesn't fall.