Not a bad start to your story I like the idea of Johnny having a twin sister and an older one. It's different, as often sibling stories that I've read have the girl being one of the others sister. Looking forward to seeing who she ends up falling for.
Word of advice, maybe take a look and work on your spacing. The chapter reads as 3 or 4 large blocks of text and therefore may be hard to follow for some readers. Double spacing and cutting paragraphs into two to three lines a piece might help, while also making sure to double space between each speakers line. For example:
"There's a shark!"
"That's not a shark, it's a dolphin."
Excuse the random statements, it was the first thing that came to mind. Hahaha. Anyways, look forward to seeing where you take this.