I too would've liked to see Sterling's reaction during Kevin's ending, but since we're left to only imagine it, here goes...
After their afternoon tryst, Ophelia feels ashamed for fucking Kevin and betraying Sterling, so she calls Kevin to tell him to never contact her again. At some point in the following weeks Ophelia realizes she is pregnant, and she probably hopes that Sterling is the father but is aware that it could've been Kevin. Either way, Ophelia wouldn't get an abortion, so she shares the news with her family and carries the pregnancy, all the while hoping the baby is Sterling's.
After nine months Ophelia gives birth to a boy, and for a while she may believe Sterling is the father, but as the baby grows and his features become more distinct, Ophelia begins to suspect it may be Kevin's child, and the family starts to suspect this as well. I'm not sure if Sterling confronts Ophelia about it first, or if she takes the initiative to tell him, but either way, a paternity test is done and it proves Sterling is not the father of Ophelia's new son.
At this point Ophelia confesses she had sex with Kevin (more than a year prior). She probably asks for sympathy and forgiveness, recalling what a crazy time it was with everyone uncontrollably horny, and she points out how she broke off contact with Kevin immediately after their affair. Sterling struggles to accept this, but as he considers her story and his own promiscuity, he forgives her. They discuss informing Kevin, which everyone else is against, but Ophelia sees it as the "right thing to do". She decides to give Kevin the chance to know his son.
Ophelia visits Kevin to inform him she had his child, which leads to their sexual reunion. She then confesses Sterling has fucked (and is still fucking) their entire family. Ophelia considers the idea of continuing a sexual relationship with Kevin while still being in one with Sterling, and Kevin has no objections.
Ophelia tells her family that she decided to continue seeing Kevin, which Brenna and Catherine react negatively to because of his reputation. Sterling would've protested also, but Ophelia would've pointed out the double-standard of Sterling's promiscuity, while assuring him that her feelings for him won't change.
As time passes, Kevin is around frequently to see his son and to fuck Ophelia. Eventually Brenna and Catherine start to see Kevin in a new light as a successful and attractive man. One thing leads to another and they end up fucking him too. All the while, Sterling is still having sex with his mother and sisters, but the women gradually start to see Kevin as the new alpha male of the house, perhaps even preferring Kevin's bigger dick.
Sterling resigns himself to the situation of sharing his family with Kevin. They may even come to an understanding, that Kevin serves as cover so the world doesn't suspect Sterling is fucking his own mother and sisters. I suspect that Sterling and Kevin might even end up tag-teaming the women, because a) the ladies still love Sterling, b) Kevin isn't jealous, and c) the women are sexually insatiable.
Most of this is conjecture, but based on dialogue from the existing scenes it seems to me that Sterling had to accept the situation. Given that, and based on the personalities of the women and Kevin, I think a sharing situation is the inevitable conclusion.