CMS and AP agree on all.
grandaunt OR great-aunt
stepmother, step-grandmother, step-great-grandmother
brother-in-law, brothers-in-law, my brother-in-law’s car, my parents-in-law’s house
grandma, grandpa, grandbaby, grandkid
granddad, granddaddy (first spelling in MW11, CMS preferred). Variants: grandad (equal variant in MW11, but does not appear in WNW), grandaddy (secondary variant in MW11, but does not appear in WNW)
aunts and uncles: great or grand?
You have two options to describe the same thing. Both are acceptable and synonymous, per both dictionaries. Note, one choice is closed and the other is hyphenated.
Your grandparent’s sibling or sibling’s grandchild is your
grandaunt / granduncle / grandniece / grandnephew
OR great-aunt / great-uncle / great-niece / great-nephew
The grandaunt option is more logical, however, as your grandaunt is your grandparent’s sister (not your great-grandparent’s sister). When you get into older generations, the great-aunt system leads to confusion. It causes your great-grandmother’s sister to be your great-great-aunt, when it is much more sensible for her to be your great-grandaunt.
Your great-grandparent’s sibling is your
great-grandaunt / great-granduncle / great-grandniece / great-grandnephew (much neater)
OR great-great-aunt / great-great-uncle / great-great-niece / great-great-nephew (ick)
You can really see the mess when you talk in genealogical shorthand: Your 3x-great-grandmother‘s sister is your 4x-great-aunt. ICK! Wouldn’t you prefer your 3x-great-grandmother‘s sister to be your 3x-great-grandaunt? It just makes sense.
My recommendation: If you have isolated references to a great-aunt, people will know what you mean, and it’s likely the term they are more familiar with. But if you refer to multiple generations and family members, be kind to your audience and use the grandaunt option for clarity. At most, the reader will think, “Hmmm, I didn’t know grandaunt was a word, but that makes a lot of sense.” Just be consistent.
Closed except with grand and great.
stepchild, stepfamily, stepparents, stepparenting
Open compound (two words).
half brother, half sister
Hyphenate. First element gets the plural, last element gets the possessive.
mother-in-law, parents-in-law, BUT my in-laws
my brother-in-law’s car
my brothers-in-law’s cars
my parents-in-law’s house
Not exactly a family word, but relevant to this discussion. Hyphenate when it means “former.”
ex-wife, ex-boyfriend, ex-son-in-law
Before an open compound, CMS says use an en dash: ex–investment banker but AP uses a hyphen: ex-investment banker.
AP specifies to not capitalize when part of a formal title: ex-President Richard Nixon. CMS does not mention this but would surely agree.
Usually an open compound, but hyphenate in adjective form.
foster parents, foster mother, foster family
the foster-parent experience
Source: CMS 16, AP 2017