So Ikaruga's mom was a mistress to MC's dad, and they had a kid. Meaning MC and Ikaruga share a half-sister.
Because of this, Ikaruga's dad had an affair and had a son, Yukiya. Maya is the daughter of that side's CEO, and Yukiya is her half-brother(through a shared mom, but no actual relation to the CEO). Presumably he's taking over the corp because girls are icky, just like MC is taking over because he happens to be the only son. Ikaruga knew about both siblings and the relationships the whole time, but MC didn't. Which is why she's alluded to wanting to beat him multiple times, and probably why MC gets random memory flashes of their shared sister sometimes when he's with Ikaruga. He might have vaguely knew half-sis when he was younger, but either didn't know she was his own sister or maybe knew she was just another sister of his but didn't know specifically who she was also related to(aka Ikaruga)
TLDR: Ikaruga isn't related to MC, but they have a shared half-sister. Yukiya is half-siblings with both Ikaruga(via dad) and Maya(via mom)