Doesn't "6th grade" in Japan refer to "end of middle school"? So that would put her what, about 13-14? I'm thinking sister and friend are somewhere in the 16-18 range by the looks of them, so it's not that huge in terms of age.
The sudden forced kiss out of nowhere was kind of a dick move though.
There's a weird movement online, where people are arguing that a high school senior is to old to date a high school freshman - junior. That it's akin to statutory rape.
I have to agree, the out of nowhere kiss was the more "wtf Mochi" moment for me. I think if there had been a buildup of the friend catching the MC staring longingly, it would have felt a lot less forced upon.