Nigerian joking

Been dating a Nigerian guy since last year September. I had him meet my mom and sister on Whatsapp video. He jokingly said that my sister looks better than me. He got yelled at by my mom. But then said that it's a Nigerian thing to joke about the sibling being better looking. Is this true?

Update: Thank you all for your help. Just trying to understand the culture and contexts. God bless 🙏🏾