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:
foster
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
Grand compounds are closed: grandmother, grandparent, granddaughter.
great
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.
half
When referring to a sibling, the compound is open: half sister, half brother.
step
Step compounds are closed, except with grand and great: stepdaughter, stepsibling, step-grandfather, step-grandparents.
How about in laws? That one gets me every time! Is it brother-in-law, for example?
Cindy,
Brother-in-law
sister-in-law
etc.
Is “adoptive mother” the same as “foster-mother”?
At least in British usage an adoptive parent is one who has legally adopted a child and has the same parental rights and responsibilities as a natural parent.
A foster parent is someone who looks after a child either on a temporary or a permanent basis. As I understand it, an adopted child usually takes the name of the adoptive parents. A foster chid doesn’t.