1) After being sent to the alternate Equestria 1000+ years ago, the sirens kept singing, waiting for a spark of equestrian magic. They have been through it all, they have seen it all. In the present times, they are Pop Stars, the Dazzlings, they love the fame, but they loved causing chaos more. After they felt what Twilight magic did to Sunset Shimmer when they were in the town for their tour, they knew they had to go there. Since Twilight couldn't help Sunset, they charged enough of their powers and went to the real Equestria, Sunset following them. Of course, they teamed up with Nightmare Moon.
2) The "pony" form of the Sirens, it's more comfortable than their real form~