Your question does not have enough information to be answered. What girls? Where? When?
Yes absolutely there is not nothing wrong with it
If they want to. Might get teased a little. Might be mistaken for gay in most societies. But there's nothing bad about a little fashion freedom.
If you are a boy, and you have a sister that wears your clothing, you ask, I can wear your clothing because you wear mine.
Yes, skinny jeans and tight hoodies and banana hammocks.yes skinny jeans and tights , skrits
They can wear what they like once they are paying for their own clothes. There are no laws that prohibit this.