A/N: I have no idea how to write an interesting summary, but I hope to come up with something better at some point. This is an answer to the "Harry Potter Re-Raising Project Challenge" issued by Lamia of the Dark.
"Sirius, you're crazy."
"Just for a few days, Regulus." Sirius' voice was hoarse, the combination of grief and anger bottled up inside him making it hard to speak without screaming out his frustration. He looked away from the infant bundled up in his arms and stared straight into his brother's eyes. "Who else am I supposed to trust right now?"
Now it was Regulus' turn to choke on his own voice. He exhaled sharply and tried to keep his composure. "And what are you going to do in these few days?"
The storm raging inside Sirius' grey eyes picked up force, and the man bared his teeth in a menacing smile. "Don't worry about it, but I can't bring little Harry with me. I'll be back as soon as possible. Please, Regulus."
"Okay... okay, I'll do it." he stepped closer to his brother and took the sleeping baby in his arms. "You know how to contact me. And... Sirius? Whatever you plan on doing, be careful."
Before anything else could be said, Regulus apparated away, leaving Sirius to stand alone in the middle of the mostly destroyed house. Dumbledore or one of his followers was bound to show up very soon, and it wouldn't be wise for them to find the two brothers together, especially when everybody assumed one of them was dead.
-x-x-x-
It took Regulus two days to find out what happened. Hiding from everybody made it difficult to keep up with the news, but before long he was forced out of hiding to procure the necessary things for Harry's well-being, and once he was out it didn't take long to hear about it.
Regulus knew the Dark Lord wasn't truly gone, and even if he did feel relief about the temporary victory over him, it was hard feeling any happiness when apparently his brother had betrayed his life-long best friends. He wasn't convinced Sirius would have ever done it, but what if it was true? And maybe contacting Regulus was also part of his plan, get the Potters killed and then hand-over his defecting little brother.
But Sirius had looked sincere that night, asking Regulus to take care of the little Harry Potter until he could do it himself.
As he entered the shabby apartment he was currently using as hiding place, Regulus sighed and shook his head, as if to shake off the million thoughts running through it. He had to focus on the task at hand first, which was feeding Harry. The kid was a quiet thing, and taking care of him had been easy for those two days, but what was he supposed to do now? Leaving him to be found by muggles was out of question, but so was approaching areas inhabited by wizards when there were still many Death Eaters around.
Taking on the role of father was definitely not something he planned on doing, but...
It had been a long time since the two Black brothers had been truly close, but after Regulus went into hiding and contacted Sirius in a desperate attempt to reconcile with him because he was sure he would inevitably die sooner rather than later, he had realized that he had never stopped loving his brother. Life tore them apart once, yet mending their relationship finally seemed possible.
And Regulus refused to let anything tear them apart a second time, no matter what might or might not have really happened. Sirius asked him to take care of his godson, and right now the infant was the only connection between them, no matter how tenuous. He might have not wanted to become anybody's caretaker, but he would make sure Harry ended up somewhere he would be cared for.
-x-x-x-
Harry Potter wasn't too bad, as far as kids went. He was generally quiet, and even when he cried because he wanted his parents it only took a little cuddling and a some magic to make him happy again - he seemed especially fond of pillows being transfigured into friendly bunnies. Regulus wasn't a particularly loving person, but he tried to think of what Sirius would have done and attempted to take care of Harry at the best of his abilities.
A month into his baby-sitter adventure, Regulus still hadn't decided on a permanent solution. He had tried to gather some information on what was going on in the wizarding world, and he was starting to suspect the fact that Harry Potter had disappeared was being kept a secret, but he also found out that the oddly-shaped scar on the boy's forehead was somehow public knowledge. Had Sirius mentioned it to Dumbledore? It made him paranoid, and it made finding an accommodation for Harry difficult.
There was nobody left who Regulus trusted enough to entrust Harry to, and knowing the boy would be recognized by any wizard in the country put the man in a tight spot. He toyed with the idea of bringing him to his mother, knowing that if nothing else she could grant him a proper wizardry upbringing. But he hadn't seen Walburga Black in two years now, and knowing she had been living alone with only Kreacher for company for almost as long at that point, he quickly discarded the idea.
-x-x-x-
Regulus heard of Bellatrix' arrest a few months after the fact. He had avoided the wizarding world for some time, but all the news about the Death Eaters arrests (and more acquittances than he was comfortable with) were the push he needed. Revealing to the world at large that he was alive still wasn't something he was ready for, but he hoped that letting his mother know would at least give him and Harry somewhere to live without having to switch hideouts every week.
He could only hope his mother's reaction to his return would be favorable. Sirius wouldn't have approved, but Sirius was gone and Regulus needed to do what he thought was best for Harry.