“There are no lines being crossed here. I have a niece and a nephew. My nephew’s mother is the sister of my niece’s father.”
Are they banging each other? If yes I would say a line has been crossed. Unless you are from West “By God” Virginia. Then it’s OK.
There are four people I mentioned here. Shall I draw you a graph of who’s related and who is not?
Six people, I mean. It also gets confusing to me.