A/N - Beta'd by Willow (The Willow Maiden), who also came up with the awesome title :)
He peered down at the sleeping form of the infant from the safety of his mother's arms, bright eyes alight with curiosity. It was obvious that he didn't know what to make of her; he was too young to understand what was happening, he just knew that his mother had gone away for a while and she'd come back with... something.
Something warm and wriggly and red, something that made a lot of noise and demanded near constant attention. It looked a bit like the toy that his cousin had carried everywhere with her the last time she came to visit; he hadn't been allowed to play with it, but he wondered if maybe he might be allowed to play with this one. If he was very careful and didn't drop it or take it outside.
"Aberforth? Ab, this is your sister, Ar-i-a-na," she sounded the name out slowly, once she was sure she had the little boy's full attention. "You're her big brother. Do you know what that means?" She paused as if waiting for him to answer; he continued to stare at her with big blue eyes, an almost comically serious expression on his face. "It means that you have to take care of her. Can you do that for Ariana and I, Aberforth?"
She took the high-pitched giggle and wide grin as a yes.
It wasn't a promise that he would ever remember, but it was one that he would adhere to none-the-less.
It was a couple years before she was old enough to play with him, and then he would take her into the back garden every day the weather permitted and they would spend hours on end in some imaginary land that their brother held no interest in. He would much rather spend his time inside; reading or helping their mother.
Aberforth and Ariana would much rather spend their time outside; and it showed in the tan lines that were a constant feature throughout the summer months and the mud they tracked through the house year-round. In the near-permanent grazes on their knees and elbows and hands; in the way that they both knew every inch of that back garden and still managed to make new discoveries every day.
"Aberforth," his mother pulled him to the side whilst Albus helped Ariana wash up for dinner. "Maybe you could spend a little more time with your brother? He'll be at Hogwarts before you know it, and you'll be wishing that you'd spent more time together."
He gave a vague, non-committal noise in response, which his mother took as acquiescence.
Secretly, he couldn't wait for his brother to go off to Hogwarts – maybe then he and Ariana would be able to go to the park without Albus, and – if he was being entirely honest – he found his brother a little annoying; but he decided to try things his mother's way – just the once, mind.
They went for a walk; they hadn't been able agree on anything that they had both wanted to do, and so their mother decided that they should compromise with a walk – he wasn't really sure what a compromise was, but he thought it must be something where neither of them got what they wanted. He would ask Albus, but he didn't really care that much.
So they went for a walk through the almost empty streets of Mould-on-the-Wold; sweating slightly in the summer heat and both sulking, thinking of things that they would rather be doing. They managed to go on for nearly an hour and a half like this, before the angry silence grew to be too much and they made the mutual decision to head back home.
"Oh, boys," their Aunt greeted them upon their arrival; she seemed subdued, as if she was trying to be cheerful for their benefit but her heart wasn't really in it. "I'm going to be staying with you until your parents get back," here her smile faltered, "I'm afraid there's been an accident. You're sister..."
They both nodded along as if they fully understood what was happening; it was pretty clear that they didn't.
That night – and every night after it until their sister's return – Aberforth dragged his mattress into his brothers room and slept on the floor; neither of them said a word about it come morning.
They tried to carry on as if nothing had happened; as if everything was still like it had been before. It was difficult, and they didn't quite manage it, but they could pretend for a little while longer. Pretend that they didn't notice the stares that followed them wherever they went, the looks ranging from pity to disgust; pretend that they didn't notice the wide berth strangers gave them as they passed; pretend that everything was fine.
There came a point, however, where pretending doesn't quite cut it; where you have to cut your losses and just accept the cold, hard truth. That day came sooner than they would have liked, but they'd all known it was coming.
"Boys," their mother sat them down in the kitchen; Ariana was playing with her doll in the corner – always in sight. "I've decided. It's time for us to leave. We'll be moving to Godric's Hollow in the morning. I trust that you will be able to pack everything you will be needing."
Silently, they both nodded once.
His brother managed to ensure that he was never known as the son of a Muggle-hater at Hogwarts; being the younger brother of a genius was a much harder label to lose.
He read to her every night – Grumble the Grubby Goat from The Tales of Beedle the Bard had always been his favourite – and he continued to do so up until the night before he left for Hogwarts. He hadn't wanted to go – she'd miss him, and he knew he'd miss her – but he didn't have a choice in the matter. His mother was adamant on that.
Ariana wasn't able to leave the house, so one of the neighbours – the only one that they'd ever spoken to – had offered to take them to the station. He wasn't sure if she knew about Ariana – and he couldn't ask her; he'd learnt quickly that Ariana and their father were not topics to ever be discussed, publicly or otherwise – but she hadn't questioned their mother's insistence that she couldn't see him off for his first year at Hogwarts.
"I'm sure you'll have a lovely time," she smiled brightly at him, obviously mistaking his reluctance to go for nervousness. "And your brother will be there to look out for you. It'll be fun," her smile widened, turning slightly nostalgic at the same time. "Like an adventure. The best adventure of your lives."
He waited until he was alone in a compartment – ensuring that no one could see him – before he let the tears fall.
He played with Ariana every summer like they used to when they were younger; out in the back garden, in the summer heat – scraped knees included.
He was furious at his brother when Albus refused to let him drop out of Hogwarts, claiming that education was important. It wasn't. Ariana was important, and Albus wouldn't be able to look after her properly. Even their mother hadn't been able to do as good a job as he had, and she wasn't bitter about missing out on the traditional around-the-world tour.
They'd stayed up yelling well into the night more times than he could count, but the outcome was always the same.
Albus just wouldn't listen.
He could never forgive his brother for what happened after; he could never forgive himself, either.