Cathy Holt was perfectly happy with her life, thank you very much. She is studying to be a surgeon, has a nice steady relationship going and is living quite happily. Then she dies. The end. Of course, that isn't what happened. Instead, she wakes up as a baby. And her older sister is some snotty kid called Petunia Evans...wait a minute. Little Petunia Dursley? Oh she is screwed
When Vivien Potter enacts Plan Omega in a fit of rage after her godfather's death, she may have acted a little hastily. The ritual may have been a little rough, and maybe she should have added the extra spell into the rune circle. It's only a few side effects. Like being decades in the past. And in a completely different body. But it's fine. Mostly.