Sonata was the only siren to hang around after the concert, and eventually met up with the Humane 6 explaining how she was only going along with the other Sirens because she had never truely experienced friendship, and only now had seen that by observing them from afar. They eventually allowed her into their group and slowly but surely, she understood friendship.
 
About a year or two passed before Princess Celestia gave her permission for Sonata to return to Equestria, where she met pony Flash. (I have a headcanon why Sirens can shapeshift from Pony to Siren in order to lure their prey) Of course, being Sonata, she didn’t recognize this immediately, but became fast friends with the stallion.
 
Not only did she tell him her horrific past, but she exposed him to her Siren form. He was surprisingly accepting, and said he loved her in any form she took. They got married soon after.
 
From this marriage, Seatide was born, a half-siren filly. Not unlike her mother, she can shapeshift from pony to siren, but her siren for is more or less an aquatic pony. This transformation is hardly intentional, usually she changes when she hits the water or in extreme distress. In pony form, she has a surfboard cutie mark.