I always was bad at all this family terms like sibling, cousine etc. I will rewrite this part to represent situation better.
About Envy, my game will not have any "trully forced" or cheating content, because I am myself not fan of such things. But, Envy have some powers, not told yet. And main character with Lust will have some powers, almost obvious which. So SOMETHING "not real" or with fake feelings/bodies can be.