Wow that Yuu controntation scene realky surprised me. I always thought Touko striving to be like her sister was bad, but Yuu really laid it out how in the end that's okay and it's just who Touko is.
Touko being like her sister is perfectly normal, that's just what younger siblings do. Trying to be her sister, on the other hand, i.e. entirely suppressing her own identity and replacing it with (her incomplete knowledge of) that of another person, is extremely unhealthy.
How can you deny her a kiss! Tsk kouhais this days
Yuu just realized that quick gratification is not something Touko needs at that point (despite both of them obviously wanting that kiss). And being the good nurse she is, Yuu used Touko's desire as leverage to make her swallow some bitter medicine, which thankfully worked as intended.