As an alternative... bait and switch. The boyfriend/girlfriend is blindfolded and therefore doesn't realize their sexual partner is being swapped for someone else. Extra points if MC comes home as sister's boyfriend is rushing out the door staring at their phone and saying they've been called away on an emergency - before suddenly doing a double take and telling you to stay away from your sister's bedroom, even if you hear her shouting (sister could be blindfolded, bound and maybe even gagged in a bit of roleplaying that was cut dramatically short - well, I say short... the sister doesn't know the boyfriend actually left).
This scenario would be rape, as would all the scenarios where you're sneaking into family members rooms and molesting them in their sleep. Maybe that's your kink, but consentual incest is a lot hotter to me.
For me it would be nice to have at least one family member have a crush on the MC. Maybe something he did in the past stood out to her - gave her a flower on valentines day, comforted her in a time of need. There's no need to corrupt her, trick her, coerce her or otherwise, but instead if the MC wishes to pursue this relationship, show her that he understands the social ramifications of an incestual relationship (needs to be discrete, treat her with respect, and go at her pace). She's the one pursuing him, but she won't make any large moves until he proves that he's got the right stuff to make it work. She's already justified incest in her head. She's into it, and she's even willing to commit to a polyamorous relationship within her family if others feel that way. The mom in MILFY City is like this, but she seems to enjoy the physical side of it more than the romantic. plus is a bit too aggressive.
Additionally, other family members that are into incest, that are not opposed to seeing other members of their family bang. A mom that wants to see her daughter and son fuck, but perhaps feels its a line she's unwilling to cross. If her sister, the MC's aunt, wants to give it a go, then she's supportive. Again, the family member is into it, has justified it, and the MC doesn't need to corrupt, coerce or trick. She is afterwards unable to resist temptatation once she sees other bang, and goes for it.
A mom or sister that feels left out once she realizes how many girls the MC is banging. They might be on the fence about incest, but the MC is attractive, and there's got to be something about him in their minds as their friends all fall under his sway. Maybe the mom hears about her son's prowess in bed after 3 of her friends all admit to sleeping with him at some point. She becomes jealous, envious, and starts to rationalize in her mind that teasing/flirting with her son is ok, but it becomes a slippery slope once he strongly reciprocates, and she can't back herself down.
