Neither of them are "rock" (not even close), but they're not terrible. I have to admit I do enjoy the songs Hillary did for Disneymania, and I liked Ashlee's Lala when it first came out (I feel ashamed now, but it can't be changed), but that's pretty much it. I can tolerate Ashlee's music as long as it keep her away from acting.
Yet neither of them comes close the Vanessa's greatness. She's an artist, and they... Well, they're not.