Family is a social construct and, as such, changes from society to society, and through time. Blood relation is usually a common denominator, but its not always so. There are cultures and tribes for example where the whole tribe is considered your family and you have to marry into other clans, or where family is only defined by the father/mother's side of the bloodline. Most modern societies define family at a nuclear level, usually adding only up to first grade cousins, but if you take a muslim society it can include for example multiple wives with their sets of children.
So, incest can come in many shapes or forms, asuming it is based on the usual definitions within the US or the western world is presumptious and often enough, wrong. So defining incest as "between family relatives" is a good way to handle it, since that actually leaves up a lot of space for different family models, concepts and structures within real or fictional universes. For example, take The Left Hand of Darkness (Ursula K. Leguin), that one has a widely different understanding of gender, family and so on, much like does A Brave New World (Aldous Huxley), so any game or content set in those universes would need to contend with those family structures within their taboos and incest rules.