The Evil Grandma
Sometimes, it’s not important who your biological parents are- the people there for you every time. Her mother and new husband were her real fathers; even though he was not her biological father, he was always at her side, and she considered him her father.
We don’t see the big deal in her calling him “dad” and his mother “grandma” – they might not be biologically related, but they act as a family! They spend Christmas together, after all, which indicates something. So what if she isn’t technically her grandma? She has a disposition that makes her sound like an evil witch.