There aren't any love-triangles, the sister of the FMC just picked out that those two are interested in each other and are either too shy (in Kubo's case) or too dense (in Shiraishi's case) to notice and act upon them. So she sets up these scenarios to 'test' the waters of how much her sister actually like Shiraishi, and not to actually commit some sort of pseudo-NTR. Saki is the younger sister of the two and notices how much Kubo is devoting a bunch of time to stuff she normally wouldn't do, and while she doesn't necessarily 'understand', she can get a read on the atmosphere and get's quite possessive of the time her sister spends. Seita, Shiraishi's younger brother is absolutely adorable.