Anonymous“is it a sin to date your cousin”
Since the very concept of sin is dependent on a certain religion, I think nobody can answer this question without making it clear what religion he (or she) is referring to.
Speaking from a christian's (roman catholic's) point of view, for instance, dating a cousin is not a sin, but it's highly frowned upon. Marriages between cousins are discouraged, so much so that one needs a special permit ("dispensation") to marry his/her cousin.
This permit would never be granted, say, to two siblings, to father & daughter, mother & son, grandfather & his niece, grandmother & her nephew (all of these relations are considered incestuous).