Do you have a sister that you love? Be thankful that you're not in a fairy tale. While brother-sister relationships tend to be chill and supportive, sister-sister relationships are competitive beyond the wildest imagination of any sitcom writer. It gets real ugly real fast.
Need proof? Just check out "One-Eye, Two-Eyes, and Three-Eyes," in which the heroine's sisters rat her out since she has a goat that provides her with food. When Three-Eyes tattles on her sister, calling her a "haughty thing" (One-Eye, Two-Eyes, and Three-Eyes.427), she sounds extremely proud of herself for depriving her sister of her only source of food. Yeah. Nice kid.
Even worse, jealous sisters are not only awful to be around, they also try to take the good sister's place in the marriage to a prince, often with the help of the stepmother. In "Brother and Sister," once the good sister is married to the king, her stepmother and stepsister sneak up on her after she's just given birth: "Together they carried the frail queen into the bathroom, put her in the tub, locked the door, and ran away, for they had made such a terrible fire in the bathroom that the queen was soon suffocated to death" (Brother and Sister.41). Talk about a truly awful way to treat your sibling.
Not every false bride is a nasty sister, though. Some women are simply trying to snag the best gent available, who happens to be engaged to someone else. This happens in "The Two Kings' Children," where the hero breaks a rule and thus forgets about his beloved, so it's NBD when his mom picks him out a new bride. Or in "The Twelve Huntsmen," a dying king guilts his son into marrying a bride he'd picked out, "and he named a certain king's daughter who was to become his wife" (The Twelve Huntsmen.244). The bride in this case was blameless; it's not like she knew that the king's son was already in love with another chick.
Alternately, the false brides are downright malicious. The bride-to-be in "The Singing, Springing Lark" ensnares the heroine's husband by sorcery, as he says, "The strange princess had cast a spell over me and made me forget you" (The Singing, Springing Lark.296). If you're a chick in a fairy tale, women who are close to you and women who are total strangers are probably out to get you, so keep your guard up. It also doesn't hurt to have some magical help.