Oh no. I hope I'm misinterpreting things here, but it looks like Saki has a crush of sorts on her cousin (saying how cute her cousin is and page 9 having her going to her cousin saying "Nagi-chan, love, love") and hates that she's close with Shiraishi (that pout at the last page). As if it wasn't bad enough with Kubo's ditzy friend giving Shiraishi crap because she thinks he's not worthy of Kubo's affections. At least with Saki, it can be interpreted as simply just familial affection towards her cousin, and I hope that's all it is, even if it'll still lead to Saki becoming something of a nuisance regarding Kubo and Shiraishi's relationship.