The Evil Grandma
It doesn’t matter who your biological parents are sometimes – your family is the people who have always been there for you. This girl grew up with her mother and new husband as her practical father, even though he wasn’t biologically related to her; she considered him her dad because of this.
We don’t see what the big deal is if she called him “dad” and his mother “grandma,” we just don’t think they’re biologically related, but they act as a family! They spend Christmas together, after all, which implies a lot. So what if she isn’t technically her grandma? She sounds like a horrible person, in my opinion.