Ondine was obvious to what happened on your side of things, since you hadn't the time to relate anything to her yet. So when you asked if she knew how to change your new tails back into legs, specifying ‘since the one who know here is unconscious after learning I'm becoming a siren’, she froze for a few minutes.
Her silence prompted you to explain that Amelia, your sister's girlfriend, happen to be a mermaid herself, one that flew to the human world after refusing to forsake her sister who, strange coincidence, is another siren.
Thankfully, your explanations were enough to help Ondine get back to herself, trying to concentrate on guiding you without falling into her military-trained mindset, it wouldn't be helpful right now. (Right now, you needed the tender care of your loving wife, not the barking of a sergeant instructor.)
She DID wonder how small your world was, but wouldn't ponder on it before everything was back under control. <Don't worry, your prior knowledge of how it feels to have legs will help you learn the trick quickly.> Ondine assured you, using your mind-link to offer a mental-hug.
Slowly, following instructions from that stranger you loved so much, your caudal fin slowly changed back into your two feet. It overjoyed you, but alas your wife needed to cut it short. <Congratulation love, you're a natural. But now come the hardest part, and that's without factoring in the pain. I don't know why it's painful to change our tail into legs when it isn't the other way around, but from now on, it's going to be hell for you.> She ended here, hopping it would be enough to prevent you from discovering that, for males, the transformation wasn't perfect, and would never truly stop being painful.
The other thing she wasn't able to hide from you, courtesy of the link, was how she wanted to help you cope with the pain. She got back in her room, undressed, and would masturbate while guiding you through the next step of retrieving your legs, sharing the pleasure she would feel with you. This made you blush and, somehow, you knew she too, was redder than a tomato.
<Now, you need to change the bones in your tails so they're identical to what you used to have in your legs. You can make your muscles adapt to that new frame, but DON'T TRY to change them back into human ones, NOT YET!>
You did as instructed. At first you only felt a mild pain and Ondine's gentle caressing herself in the most intimate area. But soon the pain intensified and OHMAHGODITHURTLIKEHELL! Even your siren sexually ravaging herself barely compensated for the pain, but it was enough to help you continue changing.
When you finally reached the end of this second phase, an exhausted Ondine said <Okay, the harder part is done. NOW you can safely change the rest of your tail.> You were as exhausted as her, but still did as instructed, which was surprisingly easy. You immediately thanked her, fearing she was seconds away from passing out.
You were about to get up, but noticed…