Is this incest or is it not incest? I'm slightly confused...

It is not incest , he is just adobted :)

SPOILER ALERT:
Not incest. They are not blood-related. Asahi is the daughter of Maria and Kisaragi Group President, Kisaragi Ichirou. Kisaragi Sakuya is the son of Kisaragi Ichirou's wife and her lover.
That is the reason why Kisaragi Ichirou doesn't love Sakuya and Sakuya's mother push him to be a perfect heir of the Kisaragi Group. After being betrayed by his wife, Kisaragi Ichirou found kindred spirit in Maria and have an affair with her resulting in Rochi (Asahi).