This isn't your FU. On a bed is a perfectly reasonable place to leave your belongings. If I left $100 on the living room table it would remain there. Nobody who I consider family or a friend would take it. For that matter nobody who I consider a decent person would take it.

Your brother and mother have problems. Problems you need to get away from. When you are old enough you need to put a safe distance between you and them. Don't be afraid to seek therapy, yourself. Your step father is trouble too but in a different way. Keeping quiet when you know something is unjust happening before you is also bad. He is likely insecure, fearful in his relationship with your mother. In a real relationship people are not afraid to disagree. I'm sure he is very kind to you but not there for you when you need it.