The Two of Us (Sweet Dreams #65)
Janet Quin-Harkin

What better way than to pose as her own twin sister and pass herself off as a rock singer? It's a crazy scheme, but she pulls it off. Sophisticated Stormy's an instant hit, and soon nobody even remembers boring old Stephanie.
Nobody except Charles. He really seemed to care about Stephanie. But what does he matter? Isn't the glamour of being Stormy enough?