Do you have any pets? I have two brothers, but I don't have any sisters.
(There is a case when you need to be emphatic that you can use the singular. A: Do I have a spot on my sweater? B: Not that I can see. A: I'm sure I saw a spot just now. Look again. B: No, really, you don't have any spots. A: Are you sure? Look right there - isn't that a spot?