The dynamic between wayne, art and senja is such that they're a messed-up ragtag little family but also wayne and art have both had sexual relations with senja and although wayne and art are uncle and nephew as art grows older they begin to see each other more as brothers but there's also a father and son type of thing going on because of the previously mentioned raising art while he was on the run from the cops. senja and art are basically brother and sister and compete for wayne's attention as well while wayne and art also compete for senja's attention, because she's a woman, and what are men to do if not fight over a woman.
And they do all this while torturing and killing people and doing hard drugs and fucking (eachother/their victims/call girls) in their shitty chevrolet el camino. And then wayne and art have to kill senja because they find she was hiding a camera in her luggage and has been collecting footage (though it was purely for her own fetishist purposes. Its still evidence and should be dealt with) . And dump her body in a ditch . Ok? Beautiful world