A day carved from rising winds artifiact confirms that Istaroth is Venti's mother. Why he still hiding his relationship with her ?

"Thousand Winds" refers to Istaroth specifically. Venti is described as one of the "threads" of the thousand winds, whatever that means.

Well we never asked him, just like no one asked paimon what she is XD 

He did say that he's related to Istaroth tho

so venti is descended from an spaceship? love it.

Artifact and glider lore seems to be meta-knowledge that is not referenced in traveler's quests as something they know, unlike certain key lore books and notes

And Venti likes to keep his secrets