She found out that Leo is a Whitelighter in season 1 episode 14 "Secrets and Guys".
He only tells Phoebe and Paige. Phoebe and Paige tell it to Piper. Leo finds out from Phoebe.
Leo Wyatt her & her sister's whitelighter.
The Legend of Sleepy Halliwell after going on a spirit quest & seeing that Wyatt will have a brother & she's pregnant
Piper finds out in 6x16 and Leo in 6x18
chris sent Leo to valhalla because he needed Leo out of the way so that he could become their new whitelighter and prevent wyatt from turning evil.
I think her name was Lilian. They were maried while Leo was serving the country, but after he died and became a whitelighter, he visited her in a dream and told her she had to go on without him, and to be happy. We only meet her in one single episode, short scenes.
The very last episode of charmed of Season 8 Episode 22 "Forever Charmed" when Piper and Leo go back in to time to save Phoebe, Paige, and even Billie
Leo is a recurring character throughout season one and the beginning of season two, while he was busy being a whitelighter and Piper was debating between Dan or Leo, but when Piper choose him over Dan he became a supporting, but always present character. That is until season eight when they had to write Leo out of part of the season for budget reasons, bringing him back for the finale.
When Chris tells them Leo and Piper have to conceive him before his birthday or he won't exist.
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.
Season 6 episode 18 "Spin City"
Season 2 "morality bites" the sisters go to the future to see why Phoebe is burnt at the stake, and piper is divorced from Leo n they have a daughter called melinda