Just my take...

It's probably already obvious to Rosetta that both Narin and Insoo likes/wants to be with Yeonhwa. So, I think Rosetta is just being nosy and trying to get a better understanding of what's really going on in Yeonhwa's head. Basically to find out whether Yeonhwa has any romantic feelings for either of the two girls.

So she's checking to see whether Yeonhwa's actions are intentionally thought out or if girlie is clueless and just going through the motions without any realization...

All that to say Rosetta's real motive is that she's dying to pounce on poor little Narin going forward.

Her newfound obsession with her is kinda wild... even though I get she comes off as a playgirl, and going after girls is probably her "normal" behaviour. I assume that this is supposed to be different from her usual playgirl shit.