Twilight's third visit to the human world involved a task that should've been undertaken the last time: finding the Sirens. Through some reconnaissance, the girls were able to locate them some distance away from the town. They were slightly surprised by what they discovered. The Sirens were living in a very nice two story house, and it was apparent that they weren't short on money.
"We've been here for a thousand years, wouldn't it be weird if we didn't have some amount of money after all that time?" Adagio explained a little coldly when questioned.
After explaining that they meant no harm and wanted to give them a second chance, the Sirens reluctantly agreed on two conditions. That they be given their singing back and be given a chance to return to Equestria.
The latter turned out to be the hardest obviously, but not just because everyone was hesitant about it. Celestia permitted them to step through the portal so as to make amends, but when all three attempted to pass, they couldn't. They received a painful shock when coming into contact with the portal. Twilight deduced that since it was Starswirl who banished them, only he could allow them to return, and he was dead as far as anypony knew. Twilight swore that she would do everything in her power to find a way to reverse what Starswirl had done, but she has yet to find anything. Before departing, the Sirens asked that she check on the status of their species.
During the time between Twilight's fourth and fifth visits, the sirens had made some good progress with making amends and being forgiven. Even at a slightly better rate than Sunset, who did her best to hide her jealousy. They were given some small leniency after explaining that what they did was in their nature, though the other students were still weary of them.
When Twilight returned for the fifth time, she informed them that sirens weren't as prevalent in Equestria as they had been a thousand years ago. They were almost regarded as legend if not for historical and scientific accounts. The best she could come up with is that they were mostly seen, albeit rarely, in the open ocean near deep waters.
This information increased the three girls certainty of the decision they had made in private. They told Twilight that staying in the human world was most likely the better option for them, though not the preferred one. After spending a thousand years here, growing more used to it and somewhat fond of it, they felt it would be a better home. Not to mention the catching up they would have to do if they returned. They were certain that a few of their species customs and practices most likely changed in their time away. They also had no idea if the longevity they had somehow retained after the battle would be affected after crossing through. They weren't exactly eager to see if they would turn to dust.
Certain aspects of the human world became a silver lining for the Sirens. Adagio had grown quite accustomed to her incredibly attractive body and using it to her advantage. She had done so in Equestria, but it was more difficult when they all basically had the same body structure. And her large mass of hair was like a crown to her.
Aria had been a moody girl in Equestria, but in the human world, there was a much greater abundance of things to enjoyably hate. Such as the Twilight novels, the Fifty Shades of Grey novels, Justin Beiber (she wet herself during his roast), just to name a few.
Sonata was fond of tacos, a few anime's, Japanese culture apparently, tacos, a skill in cooking, sharing thoughts with Pinkie, tacos, anything with Amanda Seyfried after watching Mean Girls and connecting with her character, oh yeah and tacos.
Twilight also brought with her a magic healing gem to address the Siren's first condition. She explained that with an incantation, it should bring back their singing abilities without their magical properties. And it worked, much to everyone's surprise, and the Sirens gratitude.
After their acceptance into the group, Pinkie invited them to their sleepovers every time. The first few were slightly uncomfortable, but they eventually became enjoyable. Aria turned out to be fond of sports, and would get into intense debates with Rainbow Dash and sometimes Applejack. Sonata and Pinkie had their talks of just about anything while also enjoying baking with one another. Adagio loved talking fashion with Rarity, though still wouldn't allow her access to her hair.
"Please?"
"No"
"Please?"
"No"
"Pa-le-he-he-heaaaaaaaase!"
"No one touches my hair but me, unless I allow it."
"Oh but darling, you must allow me to work on it. The possibilities are haunting my dreams."
"So you've been dreaming about me have you?" Adagio said with a suggestive smirk.
"Don't change the subject"
The back and forth of begging and rejecting happened more than once, and always ended with Rarity giving up after half an hour.
When Twilight wrote of her next visit, Pinkie decided to do something different with their sleep over.