Can you message a celebrity on Instagram ?

My sister is in love with Motley Crue she wants to talk to her idol Mick Mars on Instagram she's worried he might not write back. He probably will because she's always posting stuff about them on there sending happy birthday messages to them stuff like that either Tommmy or Nikki are always liking her posts he probably will but maybe not right away she needs to ask him something important she's going through a rough time right now she needs someone to talk to. She has really bad back problems our parents are making her wear a back brace for it she feels really embarrassed for it she feels like she's being punished for it because our dad made a law for her that she's not to go out again unless she has it on. He also has a bad back too but he probably doesn't have to wear one she said it's not fair. She wants to talk to him about it but she's very shy especially when it comes to celebrities. How can I help her ? Can you message them on Instagram or not ?