I'm on the side of those who are wary of Izumi. If she's supposedly a morally better person, she should have seen how wrong her sister had been treating her boyfriend all those years and stepped in to intervene, or at the very least comment something while keeping space (because let's be real, what person irl would want to get involved with their sibling's affairs?).
We're looking at the big sister of the most egocentric, toxic, possibly psychopathic girl in this series, and it took falling down a cliff and fucking up her ankle to finally come to grips that she lost her ex's feelings for her. If this shit runs in the family, the onee-san may be the next big bad of this arc.