You know how female dogs can get pregnant by different males in the same litter? Wellllllllllllll, a Jilypad threesome results in Lily getting pregnant with James and Sirius's babies at the same time.
It started as just a bit of fun on James and Lily's wedding night.
(Or did it start before that? Did it really start in sixth year when Sirius and Lily dated until death threats and a close call on a Hogsmeade weekend made them break up? Did it start in fifth year when James and Sirius snogged for the first time, because James had finally admitted he was curious? Did it really start in fourth year when James caught Sirius in a broom closet with Frank Longbottom's tongue in his mouth, and Alice Mayweather's hands in his trousers, making James question everything he knew about how people were "supposed to" fit together?)
It was supposed to just be a bit of fun with a trusted friend. Their "joint custody best friend", as Lily liked to joke. It's just that they had so much fun they decided to keep going. To keep inviting Sirius over for dinner, asking him to stay over and pretend that the guest room was even a consideration.
It was just fun. A lot of fun, between consenting adults, in the privacy of their own home. It wasn't anyone else's business, so there was no need to discuss it.
And then Lily came down with a stomach bug. Only it was less a stomach bug and more of a morning sickness situation. A couple of quick diagnostic charms showed that Lily was a few weeks along.
Unfortunately, there was no spell that could determine paternity prior to them being born.
"I mean, if they're Prongs' kids that'll probably be pretty obvious," Sirius muttered looking at the copy of the scan Lily had gone for at a muggle hospital.
"How do you mean?" James asked frowning at him.
"With how pale Sirius and I are, if they're his the babies might just come out translucent," Lily quipped, not looking up from the pregnancy pamphlets she was flipping through.
James snorted and dissolved in to giggles for a while after that.
They eventually settled on an agreement: when the babies were born they'd determine the paternity, and whoever was not the father would then become the godfather.
With that settled, Lily spent the rest of her pregnancy being waited on hand and foot by her boys. It didn't matter what she needed, whether it was a cup of tea, a strange midnight craving, or someone to go down on her, all she had to do was hint she wanted something and one or both of them would hop to it. Lily felt the service was almost worth the mood swings, heartburn, and getting kicked from the inside.
So, when she went in to labor late in July, she had both her boys on hand in the delivery room. They had considered going to St. Mungo's, but the hospital was not safe at that point in the war, so they chose a muggle hospital that Mary MacDonald's parents worked at. And after many hours of pacing, contractions, and other unpleasantness, the first of their twins was born.
"It's a Potter!" James said from where he was at the end of the bed, having caught the baby.
"Congratulations, James! You're a father!" Sirius said, smiling widely from his spot at Lily's side where she'd been crushing his hands.
"Looks like you're a godfather," Lily joked before wincing as another contraction hit.
A few short minutes later, Sirius was holding the first baby, marvelling at the squished face and the wildest hair a newborn could possibly have, so that James could focus on catching the next one. He knew the job was done when Lily collapsed back against the bed, but James was oddly silent for a moment. Sirius looked over, worry frissoning through him at the strange look on James' face. He was about to ask if everything was alright when James looked up, appearing a touch confused.
"So, er... I think this one might be a Black?"
"Heteropaternal superfecundation."
"That's a terrible name for a child, Sirius."
Sirius looked up from the medical text to glare at Lily. Only he couldn't hold the expression when he saw her cradling one of the twins while James was changing the other.
"You dam-- darn well know I wasn't suggesting it as a name, Lils. That's the term for when someone has a multiple pregnancy with more than one father in the mix."
"Well, I'm glad there's a muggle name for it so we don't have to obliviate the hospital staff. In the meantime, do we have names for both of our children?" Lily asked.
"I-- Yes, I know what I would like to call him. I just want to be sure that you're both okay with it."
"Well, we're not exactly about to put him back where we found him," James joked, rejoining them with a freshly diapered baby swaddled in blankets.
"I would highly object to that," Lily said, wide eyed as her imagination took her on a terrible mental adventure. She shook her head to dismiss the image and refocused on the present. "Look, the only thing that's changed is that now you're both fathers and godfathers. And quite frankly you two were probably going to get those wires crossed anyways, so I'm fine with it working out this way. Now, these babies need names. James, what are we calling your son?"
"Harry," James said proudly, looking down at the baby Lily was cradling. "He's Harry James Potter."
"Hello Harry," Lily said, pressing a soft kiss to her baby's forehead.
James walked around the bed and offered the bundle in his arms to Sirius, who held his son just as reverently as he'd held his godson.
"Who's this little fellow, then?" James asked Sirius.
"Alphie," Sirius whispered, clearing his throat after a moment. "Alphard Evans Black."
"Evans?" Lily looked at Sirius in surprise, green eyes shining.
"You and your father were close, and you told me once that you wished you could keep the name going since he had two daughters. I know you hyphenated when you married James, but I thought-- If you don't like it I can--"
"No, I love it, shut up." Lily said, sniffling.
"The different surnames isn't going to bother anyone?" Sirius asked, looking between his best friends, the two people he loved most in the world.
"Honestly, I think the different birthdays will be a bigger trip up," James said, sitting next to Lily on the bed.
"Different birthdays?" Sirius asked, looking at Lily who seemed equally confused.
"Harry was born at 11:58 PM on July 31st. Alphie was born at 12:01 on August 1st," James said with a grin.