Uchiha Ei has three siblings.
She and her twin sister were the firstborn children of Uchiha Tajima and Uchiha Sarada. From an early age, it was clear just how different Ei and Makoto were.
While physically they were often hard to tell apart, it was easy to tell who was who when spending more time with the sisters.
Ei spent all her free time–from the moment she could walk and hold a kunai–on improving her shinobi skills. Her need to protect her two young brothers, Kou and Madara, only amplified her drive to become stronger.
Makoto, on the other hand, preferred to spend her time reading and learning about their clan history. While Ei learned to fight physical conflicts, Makoto learned the art of diplomacy and how to win verbal battles.
While originally apprehensive about one of his daughter's refusal for fighting, Tajima eventually saw that there was no changing her mind. Besides, maybe it would be good for the clan to have two leaders, each with different strengths.
So, Ei was taken into battle, fighting side by side with her father, and eventually, Kou would join in as well. The boy was only two years younger than her, after all.

Uchiha Ei has five siblings.
Kou died young.
He had only just joined the generation-long war that their family was fighting against the Senju. At six years old, he was deemed old enough to be a shinobi, ready to see battle up close.
After all, his family would be there to protect him, right?
But he had gotten separated. Ei lost sight of her brother for just a moment but that was all it took for him to be overwhelmed by Senju fighters double his age.
She arrived in time to stop them from taking his eyes–useless as they would be to them–but could do nothing to save his life.
That was the day her sharingan awakened.

Uchiha Ei has four siblings.
Izuna was two years old when they entered peace negotiations with the Hyuuga clan. Ei's youngest brother would also be her last.
Her mother's health had been declining over the course of her pregnancy and then, right after she gave birth to Izuna, her heart failed her and she was gone.
She knew that Izuna never really understood that loss, he was too young to remember her after all. But Kuro, Tugakushi, and especially Madara did.
Ei and Makoto spent countless nights at their little brother's bedsides, trying to comfort them in any way they could.
Tajima had locked himself away for weeks after his wife's death. Neither of his children had been able to reach him in his grief.
Then Makoto was sent to negotiate with the Hyuuga and never returned. Of course, the byakugan wielders claimed they were innocent. Faulted the Uchiha for being too careless with their own lives.
There were no peace treaties signed between the clans that day or any day after. But it did shock Tajima out of his stupor. With the loss of one of his eldest children in mind, he threw himself into training the children he still had left.
He would do anything he could to prevent more of his family from dying.

Uchiha Ei has three siblings.
Madara was nine when Kuro died. Ei herself was twelve at this point and as clan heir had started to take command of different squads.
Some of the people in her charge were older than her but even then, her power on the battlefield didn't let them doubt her for a moment.
Still, this means she wasn't with her brothers when it happened. Ei was away, leading a battle against a group of Senju, thinking her brothers were safe at home.
Unbeknownst to her, Togakushi and Kuro had snuck out of the compound to train on their own. Ever since Makoto's death, Madara had been watching his younger siblings like a hawk. Ei did too, of course, but she had the ability to be more subtle about it.
So, while Madara was distracted fussing over Izuna, his other two brothers snuck out to escape their older brother's smothering presence.
They hadn't meant to go far, just far enough to be able to spend some time on their own. Learning new jutsus might be fun, but Madara always had the most boring way of explaining things. Surely they could figure things out on their own much faster.
Unbeknownst to them, they were not the only ones in the forest.
Madara froze when he saw the signs of a battle not too far from their home. It had already gone dark and he had been worried about his sister not having returned from battle yet. Then Izuna had started throwing up and he was focusing all his attention on his youngest sibling.
Now, however, he realized that Togakushi and Kuro were nowhere in sight and someone was setting the forest on fire.
Grabbing his gunbai–which was still too big for him but he could work around that–he sprinted in the direction of the fire.
by the time he arrived, Kuro was already dead. Togakushi was standing over his brother's body, sharingan blazing and trying to defend himself as best as possible against the bloodline hunters.
"Madara-nii!" he cried when he saw him, relief washing over him. His big brother was here and everything would be okay.
It wasn't, of course. While Madara managed to kill most opponents and make the rest flee, Kuro was still dead and Togakushi retained permanent injuries. That was the last time Madara would let his younger brothers out of his sight.

Uchiha Ei has one sibling.
Or at least that's what she thinks the moment Senju Tobiramam cut down Izuna with his sword.
Ei and Madara have spent years of their life trying to protect their siblings. After Tajima died, Ei decided to make her younger brother clan head instead. She was supposed to be cleaned with Makoto after all. She couldn't do it on her own.
But Madara could. Even though he wasn't the oldest of them, he was basically made to be clan head. It might not always seem that way, but Madara was the most level-headed out of the four of them.
Ei tended to solve most of her problems with violence, Izuna was first too young and then simply too flippant to be given any sort of responsibility.
Togakushi might have made a good alternative. Even without full use of his right arm and a constant stagger in his step, he was a great fighter and clearly surpassed Madara in diplomatic skills. If he was a little older, Ei might have made him clan head instead.
And maybe he would have been clan head one day regardless, as Madara had named him his heir (the second oldest of her siblings was determined not to ever have children). But then that all seized to matter when he was declared killed in action at age fifteen.
They never did find his body and Madara was adamant about believing he was still alive. But then days became weeks and weeks became months. Nearly a year after his disappearance, even Madara had to resign himself to the fact that his younger brother was dead.
So now, five years after Togakushi's death, Ei was sure she had lost another brother. There was no way Izuna would survive his injuries. They would bring him home and he would most likely die in his bed in a couple of days.
Only, he didn't have to.
The very same Senju bastard that nearly killed her brother was able to heal him effortlessly if they agreed to make peace between the clans. So Madara agreed to Hashirama's demand and their brother was saved.

Uchiha Ei has two siblings.
After centuries of war, the Senju and Uchiha made peace and together founded what would be known as the first shinobi village. Madara named the village Konohagakure and Hashirama became the First Hokage.
More clans eventually joined. The Akimichi, Yamanaka, and Nara had been allies for nearly as long as the clans existed. The Aburame and Inuzuka both brought a multitude of animals with them when they joined. And the Hatake, a group of farmers-turned-samurai-turned-ninja brought along what would become one of Madara's closest friends.
Shortly after the establishment of the village, Ei gained a friend as well. A handmaiden to Hashirama's wife, Yae was the only civilian able to hold her own against Ei.
Of course, Ei would later find out that Yae was, in fact, a kitsune and therefore couldn't be compared to other civilians, but the fox found it entirely too amusing to rile the Uchiha up to let her know about this fact early on.
So, years passed by. Ei founded her own squad–the Assassination and Tactical Squad known as Anbu–and grew closer to Yae over the years. She and Madara both attended Izuna's wedding to his partner of choice and eventually she started to suspect that Madara himself might have found someone he was interested in as well.
Only for him to completely upend his life in Konoha and abandon his clan with nothing but a note.

Uchiha Ei has one sibling.
Madara isn't dead, for all she knows. But he has abandoned her family and so he is as good as dead in her mind anyway.
She had just come home from a mission when she found the note laid on her dining table.

Sister.
I have been told to leave the village. I will not return. Do not look for me or question my absence.
- Madara

P.S. I will take care of him. Don't worry. -Tooma

It didn't take long for Izuna to storm into her house with a similar letter in his hand.
"Is he fucking serious?" her younger brother had asked. "He can't just leave like that! Who told him that?!"
Having no answer herself, the two siblings spent the better part of a month questioning everyone possible in Konoha but came up with nothing. Short of looking for their brother themselves, there was nothing they could do.
But Ei was the Anbu Commander, she couldn't just leave to try to find her bullheaded brother, and Izuna was similarly tied down with his new responsibilities as both father and clan head.
So years passed and while Madara was never forgotten, Ei and Izuna were forced to let go of their brother. They were sure now that he wouldn't come back. But they were going to kick his ass when they met him in the afterlife.
(They didn't know, of course, that Madara had no intention of facing his siblings in death and was entirely prepared to escape death to avoid their wrath. But that was a story for another time.)