How much of a population’s DNA needs to be mapped before you can reliably triangulate the the identity of any individual by comparing to known DNA profiles? For instance if I have two cousins on different sides in the database, can they look at my DNA and narrow down the possible matches to me and my siblings of the same sex? What about more distant relations?

(I’m thinking about how police caught that serial killer recently because some of his relatives had used one of these sites)