Why? Because as a child, he was mocked by other children, for "Ernie and Bert."
Hence he grew up hating that name, and also grew up into a bully as a defense mechanism
Satan is the witches' father
Their mother is mentioned but no father, they apparently adore Satan and he's gifted them with magic and the book and even individual powers unique to each sister. They were apparently born into being witches so perhaps each generation breeds with Satan to create the next?
- At least one character states that the Sandersons sold their souls to Satan in exchange for their powers. As for no mention of a father, just look at them. Three sisters who look nothing alike and we only hear about their mother? They most likely have different fathers.
Winnifred's book is made out of her father
It explains why he's never mentioned and her affection for it.
Allison is a descendant of the same family as Thackery and Emily
Her mother keeps the museum running and she knows a great deal about the history. also there is a family resemblance.
Allison is a distant relative of the witches.
She seems way too keen to open the spell book to check it out, as if she were confident in her own witchcraft powers. Plus, as stated above, she seems to know a great deal about the occult and her family runs the museum and keeps it in pristine condition (even to the point of a particular rat tail being in the same spot even after 300 years when it would have decayed.)
- Lampshaded by Winifred calling her a "clever little white witch" when Allison surrounds herself with salt. And in that case then, we have a true Four-Temperament Ensemble: Winifred as the choleric, Sarah the sanguine, Mary the phlegmatic and Allison the melancholic.
- It's possible Winifred was just referencing the belief some cultures had that all women posses a certain degree of magic. Allison's a good guy, so "white witch" could just be a taunt.
The Sisters became witches because society rejected them.
Related to the above edit concerning the Sanderson sisters parentage; their mother either just got around or was a prostitute and had three daughters, each with a different father. This lead to the girls being outcasts growing up and enduring the townspeople's taunts and mistreatment. They learned to keep to themselves and this being colonial Salem, rumors started to spread. Figuring their lot couldn't get any worse, they sold their souls and became witches and go about getting revenge on the people who treated them as less than dirt their whole lives. After a while they genuinely begin to enjoy evil, leading into where the movie begins.