//Sounds to me like your right, Scarlett. It's in his genes.//
From the OP:
"The thing is, he is displaying behaviour exactly like his Dad, who is bone idle, never worked and let my sister run the house, clean the house, shop, cook, raise the kids, pay for everything AND work."
Seems to me that the factors affecting his behaviour far from being genetic (he does have two parents, doesn't he?) are entirely environmental, Scarlett. He's seen a successful strategy for living off other people's efforts.
Although, frankly, Scarlett, (and please forgive me - I can't help myself) I don't give a damn.