What exactly defines kin-slaying? I mean, killing you immediate family, definitely kin-slaying. Cousins? Sure. Second cousins? Maybe. Your father’s brother’s nephew’s cousin’s former roommate? Probably not. Where does the line between kin-slaying and good ol’ regular murder go?

It’s a bit complex. 

Immediate family counts. First cousins is a bit unclear, since I couldn’t find examples of first cousins fighting each other in ASOIAF; second cousins don’t count (since Robert Baratheon killing Rhaegar isn’t considered kinslaying by anyone). 

However, foster brothers do count – which is a big part of the reason why the custom of fostering exists in order to prevent future wars by harnessing the power of taboo.