My best friend and I were always mistaken for sisters. We look nothing alike. I guess I would be "motherly" with her son, and maybe that made me look like his aunt, which would make me her sister. But I think whenever you spend enough time with someone, or if you share a close bond, you tend to adopt some of the same mannerisms, the same way siblings do because they grew up together.
It never made us uncomfortable, and we just said "nope" and left it at that. We never felt a need to explain. I think people are just asking out of curiosity or as an ice breaker to strike up conversation with two beautiful women