A random Web search suggests that people writing about families are not all on the same page when it comes to hyphenating terms for family relationships. For example:
A step-sister is the daughter of a step-parent to whom one is not biologically related.
I drew closer to my stepsister because I thought that we had something in common.
This is exactly what I loved about my grand Aunt, her passion for life and living.
My grandaunt’s husband was a businessman who ran a printing press.
Is “adoptive mother” the same as “foster-mother”?
Nakeita took Jamal back in and remains his dedicated foster mother.
My dad always speaks very highly of my great grand mother.
The sister of my great grand-mother, named Anne, married her first cousin.
My great-grandmother was a quarter Cherokee.
The Chicago Manual of Style offers these rules for family terms that include the words foster, grand, great, half, and step:
The noun forms are open: foster mother, foster father, foster parents, foster home.
The adjective forms are hyphenated: foster-home background, foster-parent role.
Grand compounds are closed: grandmother, grandparent, granddaughter.
Great compounds are hyphenated: great-grandmother, great-great-grandfather.
Note: The OED shows great-aunt and grand-aunt. M-W has great-aunt and grandaunt. Fortunately, great-aunt and grandaunt mean the same thing: “the aunt of one’s parent.” American speakers can avoid the strange compound grandaunt by sticking to great-aunt when referring to that particular relationship.
When referring to a sibling, the compound is open: half sister, half brother.
Step compounds are closed, except with grand and great: stepdaughter, stepsibling, step-grandfather, step-grandparents.