Phoebe AND Paige tell her in the episode entitled "The Courtship of Wyatt's Father" after the night spent with her and Leo in the ghostly plane. Chris actually only walks in after she's been told. He doesn't tell her himself.
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When Chris tells them Leo and Piper have to conceive him before his birthday or he won't exist.
It's 'The Legend of Sleepy Halliwell'.
When it comes to Wyatt it's between the last episode of season 4 Witch Way Now & the first episode of season 5 A Witche's Tale because Piper finds out at the end of season 4 while Leo is still frozen & by the first episode of season 5 he's making a nursery in their closet. When it's baby Chris Leo finds out in the episode Spin City with the spider demon.
The Legend of Sleepy Halliwell after going on a spirit quest & seeing that Wyatt will have a brother & she's pregnant
Season 6 Episode 16, in the end of the episode