New version - chapter 22 - right after Nina saved the MC, the custom name doesn't show in narration while he sits for Megan to look at the cuts and bruises.
Also in the private jet 'Megan' answers "I'll be fine, Megan. It's just... everything that happened, it's hard to shake it off." after Megan asked the MC he is fine. I believe it's the main character who should answer that.
During Lily handjob scene she refers to the main character by the default name too at least once.
He also calls Lily 'NINA' once when shouting he is about to cum. Might be a slip up from the the character though given that he may be having an affair with both sisters. Or it may be a mistake made when writing the dialogues. Given that Lily doesn't react i'd say it's a mistake though. I know no girl who doesn't react when you shout their sister's name while having sex with them.
Also while the MC takes off his pants while he 'sleeps' with Nina he say "OH... MY... GOSH! YES!". I may be wrong, but maybe it's Nina saying that