My theory is that Simba wasn`t Sarabi and Mufasa`s firstborn, but he was the first one to stay alive. Sarabi had stillborn cubs before Simba and has some kind of reanimation skills for those cubs who at least has chance to survive. Here she helps newborn Nuka to make his first breath since Zira is a totally unexperienced mother and lets her panic overwhelm her. And yes, the story is very much based on just fiction, I have no idea how to reanimate lion cubs and if that`s possible, and many biological processes are very simplified here

Zira`s paws were shaking, tears ran from her eyes to her cheeks and fell down onto the fur of small fluffy creature lioness held between her paws. It wasn`t breathing. Its wet fur was still covered in fresh blood and tear drops made small clean spots on it. Zira moved her tired desperate eyes to the tan lioness laying close to her and froze in silent question. Lioness sadly shook her head, standing up and revealing another cub`s motionless body.
Zira didn`t know what to feel. She was scared, in pain, broken and already lost all the faith. Her second cub and it wasn`t moving or breathing too. Newborns were so perfect for this world, too perfect it seemed, so the world just wasn`t ready to accept them. She looked at tiny reddish-brown kitten in her paws. He would be just like Scar if he ever had a chance to live. Zira breathed out loudly and let out a mournful wheeze and then started to cough very badly. Her visible ribcage moved rapidly up and down under her thin skin. Brown lioness turned her head away, she couldn`t bare to see her pride member in such pain.
Suddenly, Zira twitched and growled quietly. "There is another one" - she whispered and slowly moved away from the dead cub, taking a final glance at it. She lied on her side a little deeper in the cave and turned her back to the brown lioness, letting her quietly take the little bodies away without saying a word.
As soon as the cub was born Zira checked him out immediately. He seemed to not make any move as well and wasn`t breathing too. Something tore inside Zira`s soul. Her face turned from painful to cold and apathetic. She was just about to try and stand up, leaving her final dead cub behind, when suddenly cub weakly bumped her with his tiny paws. Zira`s eyes rounded and life has returned into her look.

"It`s alive, Udongo! What should I do?!" Zira started to panic.

Brown lioness jumped off her place and in disbelief froze for a second. Zira fell near the cub, grabbed him as gently as she could with her shaky clawy paws and started to lick it, but the cub seemed to not move again.

"No! Not again! Not you too!" Zira cried out while nuzzling the unmoving child with her nose.

"I will call for help!" Udongo snapped off from her shock and not knowing what to do ran out of the cave.

Zira turned to the place where her pride mate was standing just a second ago:
"No, don`t! They will never help! They will kill it on purpose!" She growled, but there was no one to hear her there, the lioness already ran off.

Zira didn`t know how much time passed until her pride member returned with a bunch of pridelanders. To Zira`s displeasure there was the former queen herself, Sarabi.. The lioness didn`t say a word, just for a second she glanced at the dead cubs in dark corner of the cave, and then she started to approach Zira and the cub.

"Let me help, I know what to do" she said quickly with firm but at the same time caring voice and Zira didn`t even notice how she let this lioness, her rival and enemy, take this cub, Scar`s child, into her paws. She turned the cub to its belly and started to lick the kitten intensively. Zira sat down near the lioness. Tears just ran and ran down her cheeks, but she didn`t notice, all she could ever think about now was this little fluffy ball in the paws of the older lioness. Udongo opened her mouth in tense anticipation. Another two pridelander lionesses, who were with Sarabi, standed outside. They both exchanged surprised and a little skeptical glances, but were silent.
Sarabi seemed to be even a little rude in her movements, but after just a few moments, the cub twitched, coughed and let out loud sad meow. Zira couldn`t hide her relief smile and got close to her cub as soon as Sarabi stood up. Zira took the meowing kitten in her paws and still crying snuggled the little one. Sarabi watched silently for a little while and then started to move towards the exit.

"Thank you. He is the only one who survived..." Zira whispered.

The former queen stopped and was quiet for some time.
"I did it for the little one. Not for Scar." Sarabi finally said not turning to Zira and stepped away from the cave accompanied by her two pridesisters.

Small details I`d love to add to the description:
1) Zira`s cubs are from Scar. She got pregnant while living in the Pridelands with him during his reign. Also Zira got sick while her pregnancy. Surely it wasn`t good for her cubs. She was very weak and thin, even though she ate more than other lionesses in Scar`s pride
2) Brown lioness near Zira is Udongo/Spy, the one lioness from TLG episode "Lions of the Outlands". She is one of Zira`s close friends, one of the little whom Zira trusts enough, but she is also not an experienced mother and has no idea how to treat newborn kids
3) Lionesses at the cave entrance are Kaya (left) and Kuhisi (right). Kaya is Zuri`s grandmother and Kuhisi is Leo`s mate, Tama`s mom and Tiifu`s grandmother
4) Zira`s stillborn cubs look almost the same as Vitani and Kovu, that`s why Zira stole the similar cubs from their parents. She couldn`t fully recover from that loss

2023