OK so what do you think about my cousin then? it sounds like he is just being lied too.
IF you hold with FE beliefs then you'd have to say that right now, he's being lied to - but once he gets more deeply into the subject, there is no possibility that he could not learn "The Truth" - so he'd either have to sign up to the conspiracy - or "come out" as a Flat Earther and give up astronomy, which he'd then know were a pack of lies.
There are a LOT of astronomers in the world...I don't think any of them gave it up when they discovered that they'd be lied to for all this time...so for FE to be tenable, you'd have to believe that 100% of astronomers are intellectually dishonest people who signed up to the conspiracy...and that having done that, not a single one changed their minds and revealed the truth to the world.
That's not just true of the present day - the conspiracy must go back many hundreds of years.