How would you capitalize sister-in-law as a form of address?

I'm editing a chapter of a novel translated from Chinese, where kinship terms are often used as a form of address replacing a name. In this case, the speaker is referring to another woman as "sister-in-law". I was wondering if that would be capitalized as "Sister-in-Law", "Sister-In-Law", or "Sister-in-law"?

Solution 1:

When, say, "brother" is used as a term of address (Please help me, brother!) it's not normally capitalised at all.

But based on Commander-in-Chief, I suggest you go for the "title case" version Sister-in-Law (which unlike "CamelCase" doesn't capitalise "noise words" such as prepositions). It's really the writer's choice though (or the choice of his favoured Style Guide).

For what it's worth, here's an NGram usage chart showing how usage has shifted significantly over the decades, as regards capitalising "Mother" when it's being used as a proper noun...