There’s nothing you can really do to stop people from saying these things. I would try to not be upset, because you may go off on somebody who doesn’t mean any harm. It may be someone who actually thinks 12 kids are wonderful for those who want them, but they’re so surprised at the sight that they say something and you’ll take it too personally.
Another thing that might work: “You know, people make comments like that all the time, and it my family and I really get tired of it.” Just say that, plain and simple. The average person will probably apologize and realize that, duh, they were being rude.
I have a friend who has adopted a little girl from China. While she was waiting for all the paperwork to go through, she had months to notice lots of other families with obviously adopted Chinese girls. Now, those people get tired of hearing “Is that your real daughter?” and “Is she adopted?” but my friend is very outgoing and was sooo excited about her own adoption that she naturally wanted to aproach these families and talk about the subject! But, she knew that she had to try and be tactful AND be prepared for an irritated response. She’d often say “Excuse me, but I’m in the process of adopting my daughter in China… is that your own adopted daughter?” 95% of the time, the people would light up like a Christmas tree and want to talk about it. A few people, however, said things like “No, this isn’t my daughter and she’s NOT adopted!” in an irritated tone. Anyway, my friend meant well and she wasn’t about to let that stop her from talking to the next adoptive family she saw!
So, the next time you hear “Are they ALL YOURS?” don’t necessarily assume that you know where they’re coming from! They may be about to tell you that THEY have 14 siblings and they just love to see another big family!