The term "weird sister" refers to one of the three witches in Shakespeare's play "Macbeth." Her character has become synonymous with the idea of an eerie or unusual person, leading many to search for alternative words to use in its place. Some synonyms for "weird sister" include "strange sibling," "bizarre kin," "odd sister," and "peculiar sibling." Other options include "ghostly sibling," "enigmatic sister," "mysterious kin," and "unusual sibling." Because the phrase has become somewhat outdated, there are many modern synonyms as well, such as "quirky relative," "eccentric family member," and "offbeat sibling." No matter which term you choose, they all convey a sense of strangeness or otherworldliness that is associated with the character of the weird sister.