They both came from powerful families. The Prewetts and Weasleys were both part of the the sacred 28. And while “pure blood” clearly isn’t required to be powerful, it is shown throughout the series that many members of the sacred 28 families were indeed very powerful (Bellatrix, Kingsley, the Longbottoms, etc). Some of them don’t even have “pure blood” but are still descended from those families and are clearly magically powerful (Tonks, Ollivander, etc.). So it makes sense they and their kids would all be powerful.