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

I'm not aware of any capitalization of familial names in English (aside from when used at a beginning of the sentence of course). Positions of high honor, like the president of a country of 300+ million people are capitalized, but unless you're going for hyperbole, stay lower-case for familial names.

Pertaining to the number of books I have read, I have not seen the familial terms being written in title case or camel case or capital case for the matter of fact. Though it is a noun, it is not capitalized unless for being used in specific cases, like writing the word for calligraphy purposes.

Military resistance was organized by Sidonius’s brother-in-law, Ecdicius, son of the emperor Avitus.

Hope this helps.

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).

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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...