Dear Keiva: You read me correctly. I haven't been able to find a good link about King Arthur's Harry Pottering sister. Anyway, she could only be his half sister. Uther Pendragon sired Arthur by help of magic. No recollection of reading that he did it a second time. And why didn't she use her powers to help Arthur out? At the very least, she could have helped keeping Guinevere from being naughty with Lancelot.
(He "lanced" a lot.)