I'm afraid he is just set in his ways that as the main goal, the cousin should be left as last and if he'd give her to the mc earlier everyone would loose interest. I suggested him an alternate route. To let the mc have sex with her and then in the further updates we could have further scenes with her and different locations, involve others with her in the form ot threesomes, there could be some conflict between mc and cousin that servers as a temporal cockblock which would perhaps require the involvement of another as a sidequest, etc. but got no answer.
Its pretty obvious in games like Milfy City that getting our hands on the main plot (pun totally intended here) doesn't kills interest. On the contrary, everyone wants more.