Compliment or Insult?

Every Saturday, I work-out at the Presidio YMCA for about an hour or so. While I work-out, Isaiah stays at Childwatch.  Childwatch is a program where YMCA staff will watch your child while you work out for a low fee.  

Today while I went into YMCA Childwatch to drop off Isaiah, this young lady, probably a teenager, asks me an odd question.

YMCA staff:  Are you his big sister?

Me: No, I'm his mom.

My brother and I looked each other weird, like what the hell.  This is the second time someone has asked me a question like that.

Now I'm not sure whether to take this as a compliment or an insult. What do you think?