Ah, the families of fairy tales. I have heard complaints about the lack of family members in such tales, but the explanation for that is simple.
In a fairy tale, a family member is usually a problem.
This is because fairy tales are relentless about characters. An irrelevant character may not even be mentioned -- a father with a daughter but no explanation of the mother -- and a character who has served his plot purpose may disappear without the slightest hint of why, not so much as a mention that it's mysterious. They may serve as other plot devices, but being a problem generally moves the story forward
.Stepmothers are a byword, and for good reason. Many stepmothers are indeed a grievous problem for heroines and something heroes. Straightforward murder is not out of the question, and then stewing up the child's corpse for dinner and feeding the dinner to the child's father is rather common.
On the other hand, there seldom is a reason why it's the stepmother and not the mother. Hansel and Gretel and Snow White, famously, had evil mothers in the first edition of the Brothers Grimm, only changed to a stepmother later.
Cinderella is not thus changed, but then, Cinderella and her variants do not have problem with their mothers or stepmothers. (Whatever some psychologists bent on fitting it into their model tell you.)
No, all the Cinderellas have a problem with their stepsisters -- when, indeed, they do not have a problem with their sisters. You can tell this because while there are no variants where Cinderella has only a stepmother, and no stepsisters, there are many variants where there are sisters and no mother or stepmother.
Though there are other sisters. Molly Whuppie and her sisters are thrown out by her family, and Molly saves them from the giant, and then wins them princes as husbands. Likewise, Kate Crackernuts helps her stepsister Anne after her mother cursed her (being your standard wicked stepmother). That is, they can be problems by raising the stakes for your main characters.
Likewise, brothers can raise the stakes. Hansel and Gretel hang together, of course. Hop O'My Thumb and a lot of other boys abandoned with their brothers -- or both brothers and sisters -- get to be the hero and rescue all the rest. Or a heroine has to rescue her brothers turned into birds.
Though brothers can be, like sisters and stepsisters, real problems. And they seldom have the excuse of being stepbrothers. They abandon the quest early, and then they just murder their brother to claim the prizes.
Not always, to be sure. In The Greek Princess and the Young Gardener, the oldest brother offers, that since the talking fox wants its head cut off, and the youngest brother doesn't want to, he could do it on the brother's behalf. But that is truly rare. More often they are truly useless and may even get to marry the other two princesses their younger brother rescues, and barely be bit parts at that.
Oddly enough, brothers in law tend to be helpful plot devices. They rescue you after Koschei the Deathless has chopped you to pieces and the like.
There is one other thing that a brother can do: replace a father. Just as a stepmother can be a problem, so too can your sister-in-law, though it's rarer.
When the problem is the actual brother, this also can be replacing a father. In those Cinderella variants that get classified with Catskin and its like, the heroine is not persecuted by her sisters. She is persecuted by her father who may marry her off without care, but generally wants to marry her himself. Hence, she must run away. (This rarely gets turned into a stepfather, though it's not completely unknown; it's more likely to be her brother who wants to marry her or is otherwise a problem.)
Sons may also have to run away from their fathers, usually because they freed a prisoner, but they may also get into trouble on their own and end up in another kingdom. Where their future bride dwells as the princess.
As a rule, boys get in more trouble with their fathers-in-law than girls do, and it usually happens before the wedding. The man sends him off on missions to be rid of him -- often because he is a poor boy fated to marry his daughter. Sometimes after the wedding to an animal bridegroom, he pressures his daughter. Say, staging a tournament and saying she can marry whichever knight she fancies rather than her crab husband.
Then, in that one, it was her mother, the crab's mother-in-law, who burned his shell. Mothers-in-law can be real problems after the wedding. It is generally the mother-in-law who urges her daughter to burn the animal skin or look at the bridegroom at night and thus means the curse will not break, and the heroine must chase after him for seven years or so. Or should this fall under mothers? It's never a stepmother, and her motives are generally to protect her daughter.
Not so much with the mother-in-law who is the mother of the hero. Such a mother-in-law has been known to kidnap her grandchild, smear the mother's mouth with blood, and claim she killed and ate the child. Or write to her son claiming his wives gave birth to animals and, when the son write back ordering her to be kindly treated, forge an order that the wife and child be killed.
This can also be the stepmother-in-law, who is often given more motive than the mere mother-in-law: she had a daughter, and she wanted her stepson to marry the daughter.
Though sometimes the mother-in-law is the person who receives the forged letter and goes to hide the wife and baby so successfully that her son has a hard time finding them after. Which is a way of being a problem for the best of motives.
Such are the choices of family.