Shut down nosy, inappropriate questions
Q: My wife and her sister gave birth two days apart. Six months later, my wife's sister and her husband were killed in a car accident, and we adopted their son and have raised the two boys as "twin brothers" even though biologically they're cousins.
My wife's sister's husband was a different race than us, so it's obvious from looking at the two boys that they're biologically not twins. The boys never seem to give it any thought, until adults ask nosy questions like, "Why do you boys say you're brothers when you're not?" Our family hasn't come up with the right answer to that question and I'm wondering if you could help us. Thanks.
A: What a beautiful way to deal with a terrible loss.
I can see kids being nosy about it, but adults? Really, people.
I suggest you settle on one or two non-answer answers that mark the end of the discussion (adapt as needed for your responses or the boys'):
"Because it works for us."
"Who says we aren't brothers?"
"We're not hung up on biology."
(beat) "Sorry, it always throws me that people still ask this."
"It's always the adults who ask."
"Is it important?"
"Is it important to you?"
"I'm touched that you care."
"The boys never seem to give it any thought/We don't even think about it."
"Oh, you noticed."
"Thank you for the teaching moment."
Seriously — deflect all you want, as you want, in as few syllables as you want. It's nobody's business, at all. Any follow-ups by the particularly clueless can be shut down more explicitly.