I would affirm what the others have already said.
The definitions in the Bible as Brian says are very broad, and that's where the traditional definitions that others build on come from.
The definitions in the groups for support & ministry & help for incest/sexual abuse survivors are also broad, as Joe points out with the example
of SIA, an organization & website with a 12 step emphasis worth checking out as they have lots of good literature for 12 steps.
The legal definitions become a bit more potentially problematic tho they are still varied & vague:
"The statutory crime of incest consists of either cohabitation or sexual intercourse between closely related persons, such as between parent and child or between siblings. Jurisdictions vary in their definition of what degree of relationship constitutes incest—for example, whether sex between first cousins or step-relatives is criminal. Many jurisdictions restrict the crime to acts between blood relatives—that is, people who are related by birth rather than marriage." (Encarta encyclopedia)
"sex between close relatives: sexual activity between two people who are considered, for moral and genetic reasons, too closely related to have such a relationship. Incest is regarded as a serious taboo in almost every society, although cultures differ as to the extent to which marriages are allowed between relatives." (Encarta dictionary)
"Incest: "sexual intercourse between persons too closely related to marry legally." (Webster's)
Clear as mud, right?!
Now I do think there are some distinctions made in the Bible, and in society as well. And I do think these, and certain legal distinctions, are important, for reasons I won't get into now.
But they have nothing to do with incest necessarily being any more heinous or incest survivors being any worse off as compared with any forms of SA and their survivors.
There are of course different kinds of SA & differences in them that may be of some importance
to the individual survivor.
But it's not a matter of measurement it's a matter of individual experience, needs & recovery.
I like what Marc says:
I would term it 'ABUSE'. Try not to get too caught up in the terminology. My opinion this only serves to start setting a value structure to the abuse. I.e., this was more damaging to me because this person did this and that person did that. Abuse is abuse no matter who or how often.
Here is some stuff I've picked up along the way that may be of some interest?:
Cannibalism--devouring human flesh.
Incest--devouring human spirits.
The two great taboos...
Incest--a dangerous curse, a mysterious force, that brings great evil upon & severe injury to people.
Taboo--a word I've recently seen used in titles of books on incest:
"Mother-Son Incest: The Unthinkable Broken Taboo", Hani Miletski (Good booklet; I have it.)
"The Broken Taboo: Sex in the Family", Blair Justice, Rita Justice
"Forbidden Partners: The Incest Taboo In Modern Culture", James B. Twitchell
"Incest, The Last Taboo: An Annotated Bibliography", Richard Rubin et al
"Incest Avoidance and the Incest Taboo: A Psychohistorical Study", by Miln Kaous
Hope this stuff helps, gives some good food for thot, whatever.