My stepmother is black, my father is white, and their 6-y-o twins are light-skinned with ginger hair, and dark-skinned with dark hair, respectively. My father is 75 & my stepmother 42. So nobody ever knows what to make of their family. Common assumptions are:
That my father is his wife's father / FIL & the kids' grandfather
That the twins are not brothers because their skin colour is so different
That both boys can't belong to my stepmother because one has light skin
That the boys can't be my father's because (a) he's so old (b) he's white
People ask all manner of insensitive questions. To my knowledge nobody's ever assumed my stepmother was the nanny,
but nobody seems to assume the nanny is the mother either (of at least one of the boys), because she's white.
People have a very narrow idea of what a family looks like, what makes people 'belong' to one another. It encompasses race and age and all sorts. It is awkward and unpleasant. Racist? Ageist? Certainly in that it makes assumptions about who you should (or are likely to) be with based on your skin colour or age.
Reminds me of the thread yesterday where the 44-y-o mother brought her baby in to the doctor's surgery and the nurse said 'who have we got here - obviously not mum!' Enormously hurtful when people show you they don't believe you should have the child / family you have - for whatever reason, whether it's simple lack of imagination & world experience, or prejudice.