The Halliwells never talked about it but something's been off, weird about Wyatt since his kidnapping at the hands of an unknown assailant they believed to be a demon. Said kidnapping sent Piper into labor, and she delivered her second son at the hospital with Phoebe by her side as Paige and Leo frantically and unsuccessfully searched for Wyatt. The toddler was found in the Underworld three weeks later, mostly unharmed but clearly traumatized. They took him to a magical therapist afterward and she gave Wyatt a clean bill of health, promising he wouldn't remember it as he grew up. Unfortunately, that proved to be a lie.
Leo was promoted from Whitelighter to Elder less than six months later, upon the recommendation of his mentor Gideon, and he tried his best to be there for his sons and wife. Wyatt was three when they finally realized the trauma of his kidnapping had changed him in unknown and slightly terrifying ways. "I need you to keep an eye on Wyatt for me when I can't be with him. Promise me," Piper whispered to Leo one night after Chris crawled into their bed after a nightmare.
"I'll try my best," he vowed, running his hand down Chris' back in an attempt to calm their youngest.
"I mean it, Leo. Something isn't right." She felt guilty for finally saying it out loud and wouldn't dare bring it up to her sisters - just like Paige and Phoebe wouldn't mention Wyatt's general everything to her - but she and her husband had to confide in each other.
He sighed, hating that she was telling the truth. They still wondered who had taken Wyatt and were constantly on high alert waiting for another potential kidnapping. It definitely wasn't a way to live but this was their reality now.
The boys grew, Piper and Leo keeping vigilant eyes on Wyatt, and he was eight when they finally stopped leaving Chris alone with him. They didn't trust Wyatt after Chris had been injured mysteriously way too many times to count but their youngest kept lying (or genuinely didn't know - they weren't sure, and that was even more terrifying) about how he got hurt. They were subtle about it, however.
Unfortunately, Chris fell to the wayside and Piper tried to make up for Leo focusing most of his attention on Wyatt, making sure the Twice-Blessed didn't stray from the path of good. They both felt guilty about it but they weren't sure how to handle this rather unique situation. Leo beat himself up every time Chris got upset or cried about another missed milestone. Leo squatted down to Chris' level and ruffled his youngest's hair, ignoring Wyatt's hateful glare (another terrifying aspect of his personality post kidnapping - he loved his brother but also seemed to hate him and was jealous when someone paid attention to him instead). "I'm sorry, buddy. Daddy will make this up to you, I promise. Work just kept me away." A partial truth - work had kept him away the past few days but Wyatt had disappeared again down in the Underworld and Leo had tracked him down, only to find him vanquishing demons. Piper had lied and told Chris that his brother and Leo were together, doing something Up There. The hurt that flashed in Chris' eyes nearly broke her but he couldn't know the truth - they needed to shield him to the best of their abilities. Phoebe and Paige barely knew anything either, and Piper and Leo wanted to keep it that way.
Chris looked skeptical. "You promise?"
Leo stopped himself from cringing. "I promise." Fuck, he hoped he hadn't just lied to him again.
Wyatt stomped his foot. "Leave him alone!" he shouted, and the house started shaking.
Leo's eyes widened and he scooted back. Wyatt calmed down and the house settled with him.
Wyatt was twelve when Piper and Leo finally broke and confessed everything they knew to Paige and Phoebe. Both her sisters were a bit surprised but not much - they had witnessed a lot of concerning behavior over the years themselves. "We need you to keep an eye on him when we can't."
Leo wrapped an arm around his wife, wishing there was some way they could get through to Wyatt. Maybe if they had found his kidnapper but whoever they were had disappeared altogether. "We will. You don't have to worry." Phoebe and Paige hugged Piper and Leo and that was the last they spoke of it.
And then on Chris' fourteenth birthday, everything went to hell when Piper was killed. Leo orbed in after a trip to Magic School and found his obviously traumatized youngest holding his mother and begging her to wake up. "Buddy, I need you to let go of your mom, okay?" He tried to pry Chris' fingers off Piper but the teenager refused to let go.
"Daddy, please," Chris begged, although he wasn't sure what he was pleading with him for.
Leo's heart cleaved in half even more and he nearly broke down right there but his sons needed him. He called Paige, who grabbed Phoebe after calling 911, and the three of them managed to calm down the hysterical boy. "Where's your brother, buddy?"
Chris shrugged and Phoebe hugged him in an attempt to stop him from looking at Piper's body. "Do you know where Wyatt is?" Paige pressed.
"No." Chris whirled around to face his father, suddenly furious. "Where the fuck were you?" he demanded.
Leo had lost sight of Wyatt again and he didn't want to accuse his oldest of orchestrating this whole event but he was suspicious. He refused to rise to Chris' bait, knowing he needed a fight to feel something other than the overwhelming grief. "I'm sorry I wasn't here and I'll regret that for the rest of my life. But she was gone when you got here?" he asked again.
Chris nodded.
They did eventually find the demon who killed Piper - couldn't help but brag about killing a Charmed One - and it was Wyatt who vanquished him without mercy. After his mother's death, they lost him completely.
Leo moved Chris in with Victor, knowing his father-in-law would protect him with all he had, even though he was mortal. Chris refused to open up to any of them but Victor was a much different story.
Wyatt fully embraced evil after Piper's death and there was no going back nor saving him after that. The world went to shit rather quickly under his reign.
Chris knew nothing of Wyatt's kidnapping or some of his behavior (there was no hiding the lingering oddities) when he came up with the bright idea to travel back to the past to save his brother from turning completely. That context would have greatly helped but his family was only trying to protect both boys. He and the others wouldn't learn about all the atrocities committed in the original timeline until much later.