::::::The actual definition of Incest goes beyond just sexual relations. "in⋅cest  /ˈɪnsɛst/  Show Spelled Pronunciation [in-sest]  Show IPA  
 Use incest in a Sentence 
 See web results for incest 
 See images of incest 
 –noun 1. sexual intercourse between closely related persons.  
 2. the crime of sexual intercourse, cohabitation, or marriage between persons within the degrees of consanguinity or affinity wherein marriage is legally forbidden. "http://dictionary.reference.com/browse/incest 

 Cohabitation, as defined by Wikipedia it's self is "Cohabitation is when people live together in an emotionally and/or sexually intimate relationship." It really depends on where you are getting your definition from, though, and how literally you want to take the definition. Under the above definition, the relationship could be consider incest as they do cohabitate and are emotionally involved with each other.