No. And the more attention you pay her the worse it will get. If you're married that's who should have your attention and concern. Your friend's sister will get over it.

She's fucking 11. Omg stay the hell away from her. And your other band member that you thought about hooking up with her. What the Hell? Dude she's 11 years old. She doesn't need to be hooked up with anyone. Good fucking grief.

Help her by hurling yourself off a cliff before someone else does it to you when you least expect it.