As far as I'm aware, 12-String is the only story that interacts with Sarah Carerra - and then, the characters don't interact. They're aware of each other, but they're unlikely to meet face-to-face (their styles of music are so different they're unlikely to share the same stage).

I haven't read Kira or Summer's Odyssey, but I would have thought that Robin and Alex's lives are hectic enough without attending a concert by the latest teen sensation (even if Sarah is supremely talented and a few cuts above the rest).

Besides which, Robin's more a fan of "Heedless Despair", who perform a completely different style of music. Even if the stories were set in the same 'universe', Robin and co. probably wouldn't take much notice of Sarah. "Desdemona Up In Flames" and whoever "Meaningless Death Imagery: Corporate Tools" are might possibly attract peripheral attention from Robin and co. IF they were in the same universe.

However, just because two stories are set in the same country at the same time, it doesn't necessarily mean they're in the same 'universe', or even if they were, it doesn't necessarily mean the characters are aware the others exist. Take a real life example: most people have probably heard of Stefani Germanotta (or at least her stage name), a smaller group have heard her songs, a smaller group still would like the songs enough to buy a copy, and an even smaller group would have the time / resources / inclination to attend one of her concerts.