Good point about the awkwardness.The awkwardness is probably part of the appeal of these sorts of movies. They are crossing some serious boundaries after all.
Although it doesn't look like the sisters in AVOP-162 actually interact much with each other, judging from the screens. Also, one sister is noticeably less attractive than the other.
Just shows that to find hot sister-sister or mother-daughter pairs that actually do stuff to each other is very difficult.