It's a movie about a girl who I think wished her brother or sister away and then some trolls or goblins came and got the baby and took the baby to this crazy world through her mirror I think and it was a man who had the baby she had to do certain adventurerous things to get the baby back the man was like a rock star or something. It was a movie made for kids in the late 80's or 90's please help me figure this movie out thank you.

It vaguely sounds like,.

Labyrinth

A teenage girl has to baby-sit her cranky baby brother for an evening. He's crying and she's at her wits end. She started to tell a story about a little boy taken away by goblins. She wishes that the goblin king would take her brother away. Someone was listening, because at that moment,the lights went out, and when they came on again her baby brother was gone.
The goblin king appears in the nursery and tells the young woman that if she wants her brother back she'll have to go to his goblin kingdom and make it through the dangerous maze surrounding his castle. If she fails to do it in time her baby brother will become a goblin.

-view trailer-

I agree with straydog, that definitely sounds like Labyrinth. David Bowie is the Goblin King, that's where the rockstar thing comes into play.

Definitely labrynth