You can't convince anyone to get rid of their stuff. You can only lead by example and keep the communal areas of the home clutter-free and welcoming in the hope that other family members will follow suit. Keeping laundry to a minimum is critical to keep clutter at bay. Follow my laundry steps here if washing is taking over your home. Keeping the family's clothes to a minimum and creating a kids capsule wardrobe helped me stay clutter-free and reduced the amount of laundry I had in the home and kids' rooms. You can read how I created a kids capsule wardrobe here.