They first said it in third year when nobody could help but notice that Sirius had come back from the summer holidays even more beautiful than when he'd left. He was still loud and obnoxious and hung around with Potter all the time, and Lily still didn't like him, but she wasn't blind. Mary caught her watching him one day, and whispered in her ear, "be careful of Sirius, Lily. I've heard he's been out with half the girls in our year, bit of a womaniser." Lily had never seen him with anyone that wasn't Potter or Pettigrew or Lupin, but she glared in his direction anyway and decided to stop looking.
They said it again in fifth year. Sirius had fallen out with the other boys, heavens knew why, and he'd taken to sitting beside her in Charms. She didn't want to admit it, but sad and lonely, he was quite nice away from Potter, even if he did flirt with everyone that moved. Lily knew she'd fall for him long before she did. Dorcas seemed to know too. "Be careful of Sirius Lily. He's slept with most of the girls in the school, you know what he's like." Lily suspected this wasn't strictly true but said nothing, and let him go when he went back to his boys.
The third was in sixth year, within their first week. They'd been partnered up together in Transfiguration after far too much hilarity between Potter and him. If he'd been beautiful in third year, he was a god now, all shiny black hair and long lean lines, and shit if he wasn't also really, completely lovely, even sometimes when he was with Potter. Lily was falling by now, but she did try to stop herself. She didn't want to love him, she'd seen the way he looked at Potter, all love and longing and adoration. She would never compete. "Be careful of Sirius, Lily," Marlene told her one day after class, though she sounded resigned. "You've seen the way he is with Potter."
By the fourth time, Potter was James and Lily was Lily, and really, she finally saw what everyone else did in him. She still saw what she'd always seen in Sirius too, but he seemed to only see James. Sometimes, she thought James only saw Sirius, but he'd been overjoyed when she'd asked him out, and for a while he only looked at her. She still looked at Sirius though, and saw the heartbreak he was normally so good at hiding. Peter saw that, if nothing else. "Be careful of Sirius, Lily. He can get destructive."
They hadn't even lasted a fortnight without James crying over Sirius, and Lily finally let herself go and cried over him too. When Sirius, smiling but hesitant, suggested a solution, Remus was the one to warn her, for the fifth time. "Be careful of Sirius, Lily." And that's all he'd said, and he'd smiled when he caught the three of them lounging together on James' bed every night for the rest of the year. Lily had smiled too.
It was completely different when James said it. "Be careful of Sirius, Lily." He laughed as Lily squeezed her boyfriend, pressing her pregnant belly against his back, reaching around Sirius to pull her husband closer to him too. "You'll kill him." She nuzzled Sirius' neck and grinned onto his skin, pointing out that that was the idea. The three of them laughed, and were beautiful. Lily loved her boys, and her boys loved her, maybe as much as they loved each other.