Marital sex just needs to be varied. If the couple won't do it together, then clearly they will still do it secretly each separately! I think this home variation is acceptable, in any case it is not as extraordinary as entertaining a large group of swingers. My wife and I once invited me to one of these, against her this clip is just sex puritanical family!
Found a way to bring the relationship between relatives closer. The sister joined her brother and lecherous wife in bed after their proposal, immediately spreading her legs and letting them direct the process.