Since the game has two distinct routes, development of each is welcomed. For the MC-centered story, getting your family to act not-family like should build on each of their quirks. The MC should use the gang to gain power and also control of the ladies. His gang power should be used to eliminate some competition, but also to help his step Dad close the case at the end. Step Dad will, of course, have to accept the new living arrangements.
If you run the 'NTR' route, there should be many similarities. The ladies should be seduced (not drugged!!!!!!!) into the seedy world of the gang by Davide taking advantage of their individual issues. A flashback to how he bagged Mom originally would be a nice touch (by the dev). Some smooth lines, to accompany that horse cock, needs to be in Davide's arsenal. A gradual seduction makes this route that much better, not just drug and fuck.
This route doesn't need to exclude the MC. He should get action as it comes, but also building up his power in the organization to rid himself of Davide in particular. This way the two storylines can converge. Of course, bad endings should be available for the pathetic losers out there!