Naw, she clearly knows they're keeping it a secret.This might be where their relationship loses its secrecy: Rin going on about the "cool onee-chan her brother's dating".
I mean, "cool and mature" and "horny" are pretty much synonyms to a lot of people lolI don't think they'll go this way but itd be wild if bro gets anxious about his little sister wanting to be like his perverted gf. That finger suck wasn't "cool and mature" it was just horny lol.