Mrs M,
OK at this point you probably aren't reading this thread anymore or just skimming it quickly because everybody has pretty much piled on and its not fun reading a whole bunch of people taking you to task.
Your real feeling about this is that you are annoyed that your oversensitive OCD niece made a big deal about a harmless joke. I think this might be a good thread for you to gain some insight. I do agree the "joke" might have been a very minor slight (I mean its not like you spread vicious gossip about your niece or anything like that) but your understanding and reaction to the situation reflects your need to try and empathize with others more.
Here's a typical read of the situation by you (your comments):
, "I can admit the remark could be perceived as thoughtless." (Could be perceived?? - it was thoughtless, just admit it)
"My husband did make the joke, and his intention was as a joke too" (your husband did not post it publicly - it was all you)
"I regret that it hurt my relative feelings",(my relative? she's your niece for god's sake, most of her life she's seen you as the adult- also you "regret" but you said later you feel no guilt)
"but I cannot accept that remark was made in an intentionally mean-spirited context, because it was, in fact, not". (I don't know if I trust your instincts on this - you have expressed some hostility toward your niece)
I also wonder about your relationship with your sibling that's her parent. If I did that I would call my sister/brother and apologize to them too. You don't mention them.
Mrs. M, this is more brutal than I usually am but you needed a wakeup call because you have some significant blind spots to your own behavior.
I just want you to accept that you are dealing with your young niece, you are the mature adult, you messed up, you hurt her feelings, and you should feel quilty.