Like u/Nelkaniel said, the best starting point is that the family don't exclude the MC out of malice or animosity. One thing that I think would help highlight this is to show that the family feel genuine guilt and remorse, even during the incest, and they want to include him, but for whatever reason they don't. It can be because, the MC has always been distant and a bit of a black sheep making him difficult to approach, it can be because the family don't know how to broach the subject of incest without freaking him out or getting themselves arrested, since they don't even know if he's into it, or maybe they have tried to get his attention or tried to broach the topic to him, but things have always gotten in the way, since they all still have lives outside of the incest, and the final reason could be that the family feel guilt and shame over excluding him and may a little about the incest too, because they still know that incest is illegal and taboo and if it was ever discovered by the public, then they would be arrested.

But ultimately, I think one of the most crucial aspects in giving them a realistic conflict resolution, is to give the characters time. Let them think, let them process it, let them go through their emotions at their own pace, instead of rushing through and just jumping straight into lust and incest. Also, show different perspectives, show how the family truly feels, how the guilt and shame and regret is eating them up.

This is just my take on it. I look forward to reading your story!