Well,
first the actor and actress have to be compatible, so the sex has to be sexy.
Then the pretense of family is important, so sneaking around a supporting cast family member, or reference to an absent family member, or going to a love motel out in the city, or one of the couple insisting they're family helps quite a bit.
Lastly the man should be the initiator, the woman should follow.
This part basically every western porn fails miserably. You're not trying to break gender norms here, you're just trying to twist family norms just enough that the family member replaces the lover role secretly, like the daughter replacing the wife in secret, or the sister becoming the girlfriend in secret.