Most hurtful

I think the most hurtful thing that has ever been said to me about adoption was actually said to me by my 7 year old nephew. "You're not Cayden's real mom."
I guess the first time I heard it I was a little taken off guard but didn't take offense to him saying it. Tonight when I heard it for the 5th time, completely out of the blue, I had to walk out of the room and broke down bawling... My sister-in-law said I needed to explain or to him.  I did, the first time he said it to me.  You see the situation in their family is a little different.  Their oldest is from my sister-in-laws first marriage.  However my brother and sister-in-law have been married since their oldest was 3.  So she told me to tell him that his oldest sister's dad isn't his real dad but she still calls him dad... Sorry but that's completely different. And to be completely honest I don't think I should have to anymore. The first time he said it to me noone else was around so yes, I did explain it to me (as well as the second, third, and forth time).  I think when the parent it their it should be their place to correct their child. Am I wrong?
I then asked my mom how to address it and was told "Oh get over it."  that sent me over the edge!  Why is it that my family doesn't seem to care?  They don't seem to correct the problem.  Yes he's 7 but what he is saying is wrong.  It should be corrected by someone other than me because clearly me correcting him isn't working.  I freaked out and said something I shouldn't have.  I was just asking for support, help, answers.  Instead I got nothing.  I'm frustrated and hurt.  I know I need to say something but honestly right now I can't talk/write about it without crying.  My biggest fear is that people will say that to Cayden.  He's so sensitive that I fear it'll really hurt his feelings.  This is one area of adoption that really isn't all that wonderful.