We are starting with a big one: It is a frequent complaint that alternative family models are not represented in a positive light in most traditional tales.
Stepmothers are almost always evil (even according to top folklore experts like Maria Tatar!), and adoption either happens with no biological family in sight (e.g. children just miraculously appear), or it is part of the happy ending that the child returns to his/her "real" family and fulfills his "true" destiny. While these motifs also have their symbolic meanings, they don't exactly sit well with adoptive- and step-parents who want to build a storytelling relationship with their children (or the other way around).
So, here are some stories with positive portrayals of adoption and stepmothers:
Hilde, the Good Stepmother
An Icelandic fairy tale about a princess who is cursed by her mother at birth - she is destined to get pregnant out of wedlock, kill a man, and burn her father's castle. After her mother's death, the king intends to marry again; Hilde, his new bride, befriends her stepdaughter, and helps her fulfill each point of the curse while avoiding disaster.
(You can read this story in All the world's reward here)
The Lion's Whiskers
An Ethiopian/Amhara tale. A mother hopes to win her stepson's love by asking a wise man for a potion. While gathering the ingredients for the potion - the whiskers of a ferocious lion -, she learns a valuable lesson about patience and persistence, and figures out how to gain her stepson's trust without any magic at all.
(You can read a re-telling of this story here, or read it in this book, or this one)
The Fishwife and the Changeling
A gorgeous Scottish folktale about a woman whose child is replaced by a changeling. When she figures out the swap, she forces the Fairy Woman to give her own baby back - but also keeps the changeling, and raises the two boys together! When the fae boy grows up he has to make a decision whether he goes back to the fairies, or stays with his mortal family. He chooses the people who raised him. One of my ever favorite tales, and also the only positive changeling story I know.
(You can find it in Folk tales from moor and mountain)
One of the most famous Welsh legends, the bard Taliesin's story begins with being fished out of a river and adopted by a chief's clumsy son and heir, Elphin. When he grows up, he saves his adoptive father's life and his mother's reputation.
(Actually the story begins way earlier than that, but it would be too long to summarize)
(Read the story here)
The Serpent Mother
A folktale from India about a poor girl who marries into a rich family, but has no relatives of her own. When it is time to celebrate her first pregnancy, she makes friends with the Snake Mother and her people, and they accept her as one of their own, and shield her from her in-laws' bullying.
(Read the tale in Folktales from India)
This Scottish fairy tale will also come up later - it features a mean stepmother, but also a kind stepsister who loves her sibling despite her mother's jealousy.
(Read about it here)
Cambodian folktale. A boy's entire family is massacred out of prejudice, and he only survives because a kind-hearted man hides him and a merchant adopts him. He grows up to be a famous scholar.
(Read the story in this selection from the Gatiloke)
This gorgeous fairy tale from Brittany tells about a five-year-old boy found and adopted by a kind chief, and named "I don't know" (N'oun Doaré) because nothing is known about his origins. He grows up to be a hero, with a rusted sword and an unlikely mare for companions, rescues a princess, and marries his horse. (No, really.) (Okay, so the horse turns into a princess first. SPOILERS.)
At the end of the story, he is offered the truth about his birth parents - but he refuses, claiming that his adoptive family is the real one for him.
(Read it in Celtic myths and legends)
In addition, there are also several positive legendary and mythical adoptions one can think of: King Arthur being raised by Sir Ector; Persian hero Zal being raised and protected by his bird-mother the Simurgh; Fionn Mac Cool being raised by two women (and he turned out mighty fine); Aslög, Brünhilde and Sigurd's daughter, being saved and raised by her foster-father. I also found several adoption stories in this amazing collection of Thai folktales.
Rachel Hedman in the ETSU Storytelling program wrote her thesis on adoption folktales. You can check it out here!
Are there any stories you can think of that I didn't mention? Let me know! :)
(picture by Lucas Gama on Deviantart)