Assumptions on who's the mother.

[Calistoga]

It sounds like you're handling it very well. It doesn't matter how easy a mistake it is to make, people will always feel a little silly. Smile and tell them it happens all the time, and they should feel a little better. As long as you get it right off the bat, embarrassment should be short lived.

[Zilla]

I am 14 and 15 years older than my siblings and got this assumptions all the time.  I just did what you said, "Oh no, these are my siblings.  She is our mother." and leave it at that.  They will get embarrassed, there is nothing to do to avert that.  It isn't like you are getting upset or being snide with them.  Just say it cheerfully and move on.

[Piratelvr1121]

Reminds me of when my parents and brother took a trip with my older two years ago when my brother was still a teenager, so about 9 years or so ago.  They said depending on the areas they were driving through, when they stopped it was either assumed that my parents (in their 50s) or my brother (17) was the parent.   Either time it was explained the boys were grandchildren/nephews.