His sister asked him

It seems that their parents found out she got very drunk at a friend party so they grounded her for a month. On one Friday she couldn't go out, her parents had a social dinner and her brother was planning on spending a night at a friend's but she didn't want to stay home alone so she asked him if she could stay with her. He said that he could but, only if she promised him more fun that at his friend's house. She said he wouldn't have this kind of fun there...