I would check to see if your sister/brother will appreciate
cut-resistant gloves, and maybe a chef's knife - Sparky loves his, and they make me a little less worried about teaching him knife skills. Or you might try a traditional BBQ apron/mitts set (Sparky also loves his arm-length BBQ gloves) He also has a Chef's hat that he enjoys wearing on occasion, but that might not fly with an 11-year-old.
What kind of a cook is your nephew - is he more of a baker or more of a chef? A really nice mixing bowl set would be appreciated by a novice baker, a safe-to-use chopping device (the kind where you put the food in a cup, put the device over the top, and push down repeatedly) might be appreciated by a small chef (even though I don't like those.) Sturdy, colorful silicon tools -whisk, spatula, tongs - might be a good way to go in either case -
these are cute.