The Williams de Beaumont family is an extremely powerful family consisting of various supernaturals, including Witches, Whitelighters (Fae), and Vampires. The family was previous foundered by Dorothy Blanchard de Beaumont, who's daughter is the current great grandmother of the surviving generation. Her two children were enemies, due to their mothers prophesy of the 'Power of 3' that upon the arrival of 3 siblings they'd rule Sunset Valley and protect it's citizens with the siblings believing their own descendants should rule.
The Power of 3 prophesy was believed to have been false due to the arrival of more cousins. Dorothy had 2 children, Judith and Delyn. Judith's children are Danielle, Lauren and Kyle. Danielle has a different father to her siblings which makes her a half-sibling. Her children are Jordan and Katie. Lauren's children are Evie and Noah, however they have the same father as Jordan and Katie making them half cousins, and half siblings. Kyle's children are Tyler, Finely and Ché.
Due to Jordan and Katie only being Evie and Noah's half cousins due to their parents being half siblings, and them sharing the same father, this makes them half siblings so that they equate to 1 'full sibling' allowing them access to the Power of 3. They're known as the Williams Coven, while Tyler, Finley and Che are known as the Tori Coven.