Dodging accidental incest is basically the most popular sport where I grew up.

Joking aside, where I grew up there were certain "clans" as we only somewhat jokingly refered to them. Basically large interconnected family units that were usually dominated by a single central family with smaller branch off families on the periphery. Dating someone within your clan wasn't necissarily off limits because that person may not actually be related to you, but if you were in the same clan then you knew your families were very closely linked and you have to be careful. If you wanted to be safe though then you just date someone from outside your clan. Basically if you mention the last name of that central family and they don't recognize it, then you're usually good; if they do recognize it then you need to do some more digging.