How common is it for families to lie about family history/relationships?

Like your mom telling you that the reason you have 30 year old uncle and you’re 28 is because her mom (my grandmother) got pregnant when my mom was 18-19 years old. Or maybe they’ll tell you that you have all these cousins but what if a cousin is a sister or a brother? I’ve been watching a lot to life time movies and it has me wondering lol.

How common is it for families to lie about family history/relationships?
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