Well....in my mind the only way for this to follow wrote is for the MC to receive a call that his father has died and that despite already being upper middle class, his father was worth many millions so he's now very rich. Despite his hot, current wife, step-daughter, and other various on-the-side hotties, he leaves them all but has to move in the mansion with his suddenly nice mother and his two sisters. One sister being all smoochie and submissive, the other sister being all RBF and mean. And wait...he's also lucky that for some unknown reason they (his siblings) never got married or moved away.
Then, as it should be, one night they all see him sans clothes while he sleeps. They are immediately and forever transfixed by his wittle pee-pee to the point they they have no jealously towards each other and agree to share their brother as their number one. Then one of the sisters tells a friend or two about the aforementioned magical weenus and now the MCs harem has increased from 3 to five and beyond belief, none of them mind the other.
Well....that's how it
should go.
EDIT
heh...this was a joke...sarcasm. I need to sign the two of you up for comedy 101 class.