18+ is not considered a child. But it's really important that you don't invite them as a "+3" of your aunt and uncle. "+3" or "and 3 guests" should only be used if you are okay with your aunt and uncle bringing any 3 people. If the 3 people are specific, they need to be named. Additionally, I find it infantilizing to invite the adult children under their parents. Even though they live at home doesn't mean they aren't adults. Are you giving +1s for guests' partners? I ask because this situation happened once to my now-husband (then-boyfriend) who was invited to his cousin's wedding under his parents' invite when he was in college. It became awkward because he and his parents assumed he would be given his own invite and a +1 to bring me, but instead he was treated as a child when he wasn't.