Sort of reacting to the previous chapters as well, but isn't this actually pretty straightforward?
Mochizuki likes MC. MC is trying to mend their relationship under the guise of healing their familial bond but doesn't realize he likes his sister romantically.
He's trying to "make his sister understand [about them being family]", Mochizuki is doing exactly what she said from the beginning: making both of them understand [about them being family, and nothing more, by firmly establishing that their relationship and feelings can never go any further].
By helping him establishing his sister firmly into the "sister" spot, Mochizuki automatically clears the "romantic" spot for herself.