Hear me out, this might take you to suspend your disbelief that people talk to their opposite sex siblings for reasons other than wanting to fuck them (I know! who would ever!) but maybe just maybe Touko dislikes Mao because both Touko and Yuuto have been effectively friendless shut-ins for most of their lives with each other being the closest relationship either have had, and the nearest thing either have had to friends, then along comes Mao who is the reason Yuuko started spending less time with Touko and eventually moved out of the house and reduced all contact to a few line messages and ocassionally dropping by the house without warning to pick up some of his shit (something his mom actually calls him out on THIS chapter) so they spend very little time together anymore.
It's entirely possible Touko is upset with Mao over leaving her with basically no peer or near peer companions until Tabata shows up.