Obviously, Jaune's sister Saphron had not been conceived of when the show started. But I love the idea of her visiting Beacon in Volumes 1-3 and asking Jaune to babysit little Adrian while she goes and does... Stuff.
Jaune; sigh "She handed me off to random family members when I was a baby too, kid."
Adrian: "Apuh!"
So Jaune lugs his baby nephew around Beacon, feeding him, taking care of him, and perhaps getting into misadventures/child endangerment.
- - -
Jaune sighed as he entered Combat Class. He could feel Professor Goodwitch's eyes on him, as intense and probing as an X-Ray machine. Adrian, given he was a baby, just babbled obliviously as his wide eager eyes looked around at the combat class from the baby carrier he was strapped into against Jaune's chest.
"My sister needed a babysitter and dumped my nephew on me again, Professor," Jaune said, "I swear he won't be any trouble."
"See that he isn't," Goodwitch stated in her usual calm, controlled tone. He walked over to his seat with Teams JNPR and RWBY, and unstrapped his nephew. He held him in his lap, and pulled out a pacifier. The baby whined a bit, but began to suckle contently as his uncle rocked him.
"You're very good at that, Jaune," Pyrrha said with a warm smile.
"Seven sisters," Jaune replied blandly. Adrian grasped onto Jaune's tie, and without looking, Jaune carefully untangled himself from his nephew and slid a baby toy into his fist instead, "You pick things up."
Weiss was already moving to try and sit next to Jaune, but surprisingly, Yang beat her to the seat. She scowled at the busty brawler as Yang cooed over Adrian.
"Aw... He's so cute," Yang murmured, "What's his dad like?"
"Don't know," Jaune said with a shrug, "My sister married a woman and they got an anonymous sperm donor."
"Really?" Ruby asked, peering over Jaune's shoulder, "Cause he looks a lot like you, Jaune!"
Jaune shrugged.
"My sister's wife carried him. I guess they got a good match?" He stroked Adrian's head, frowning. Funny enough, he had donated to the sperm bank for 'medical testing purposes' when he turned 16. Could it be that...?
Nah. That was ridiculous. Surely his sister and sister-in-law would have told him, right?
- - -
Somewhere, Saphron and Terra Cotta-Arc sneezed.
- - -
Still, maybe since he had his nephew, he'd get out of fighting-
"Jaune Arc, you will fight Cardin Winchester," Goodwitch stated firmly.
Jaune sighed. Of course, he thought to himself.
"Hey, uh-"
"I've got him," Yang said quickly, before Pyrrha could. She took little Adrian into her arms and rocked him. The baby squirmed a little, but settled down against Yang's chest. She beamed at him.
"Go get 'em, Daddy!"
"Thanks," Jaune deadpanned. He looked at Pyrrha, who smiled encouragingly.
"Don't fight for yourself," she said softly, "Focus on fighting for your nephew."
Jaune looked into his nephew's bright, innocent eyes. How he stared at him with open happiness. He smiled back.
"Yeah... Thanks Pyrrha, I will," Jaune said.
- - -
He'd gone to the locker room, changed, and entered the ring. Cardin faced him, the taller boy, as ever, ready to fight. Sure they'd sorted things out at Forever Fall (kind of), but he was still a dick.
That said... Jaune realized he was going to have to fight Cardin until he beat him. He glanced over at his baby nephew, looking at him as he sat in Yang's lap. Weiss, Pyrrha, and Ruby fussed over him, but the baby kept his eyes on him.
Jaune took a deep breath and got into his fighting stance.
"You will fight until your Aura falls into the red or ring out," Goodwitch stated, "Begin!"
Jaune immediately went on the offensive, just as Pyrrha taught him. He closed in and got in Cardin's guard, striking hard and fast! Cardin swung his mace around but Jaune ducked and got in a hit!
Cardin fell back, but rallied with an aggressive series of strikes, trying to force Jaune back. Again, he used his smaller size and improved footwork to dance around the blows. One hit his shield, but Jaune angled his shield to disperse most of the momentum, and used the gap to get in another hit!
"Ugh!" Cardin cried, swinging his fist at Jaune. Jaune fell back as he took the hit to his cheek, again grateful for training with his team and Pyrrha. That hit hurt, but he was still on his feet.
The fight went on, Jaune just barely able to see his aura level on the main screen overhead while he kept up his assault. Cardin was, for the first time, much lower on Aura than him! Cardin was even breathing hard, angry, incredulous at how things were going!
Cardin tried to unleash a blast of Fire Dust from his mace, to get a hit on Jaune, but he used his shield to bash the mace away. Even as the Fire Dust exploded deafeningly loud next to him, he swung his sword. Cardin tried to deflect it with his gauntlet, and caught the blade on the metal. But Jaune recalled Pyrrha's training, bent his elbow, and rammed it into Cardin's side. He then twisted his hips and slammed his pommel up into Cardin's chin!
The taller young man grunted and fell back. Jaune held back a grin. He was close, so close, he was going to win, for the first time he was going to win-!
"WAAAAAAHHHH! WAAAAAAHHHHH!"
Even over the ringing in his ears, he could make out Adrian's cry. He glanced over out of the ring: His nephew was crying, screaming in fear or need. His teammates were trying to calm him, with waving toys, funny faces, or rubbing his tummy-But Adrian wasn't having any of it!
Jaune glanced at Cardin. He was still staggered, on his last ropes...
Adrian cried again.
"I forfeit!" Jaune said quickly, running and jumping over the ring railing. He put down his sword and shield as he rushed over to his crying nephew. Yang gaped at him in shock as he pulled Adrian into his arms.
"There there, I'm here, it's okay," Jaune murmured soothingly, rubbing Adrian's back and head as the baby cried into his chest. The baby boy calmed down, his whimpers vanishing as he clung to Jaune's chest.
"There you go, everything's okay, buddy," Jaune whispered, kissing Adrian's head. "There you go, you're okay, aren't you buddy?"
Adrian sniffled and Jaune pulled him up so he could nuzzle against Jaune's throat. Jaune chuckled, and nuzzled his baby nephew.
"There you go... There you go..."
He finally looked up... And flushed. Everyone in class was staring at him. The girls in particular were giving him... Almost gooey looks? Some of the boys looked approving, in a manly sort of way. Ren included.
"... Winner, Cardin Winchester," Professor Goodwitch said, in a softer tone than she normally used.
Cardin groaned as he staggered up to his feet. He scowled over at Jaune, and sighed.
"Yeah, sure I did," he muttered, sounding depressed.
- - -
Obvious inspiration is obvious. Vendetta543's fic "Jaune's Baby-Making Arc". But it was too cute not to do.