User blog comment:OneofthePendragon94/The age of Merlin, Arthur, Morgana and Gwen./@comment-123.231.120.248-20200528061022

in "beauty and the beast" Uther says he hadn't been with a woman since Ygraine's death. As she died in childbirth there can't be any younger siblings for Arthur. In "crystal cave" Uther says Morgana is his daughter with Vivene. So she should be older than Arthur.