So who did your other sister (the one getting married) pick as her maid-of-honor. If it was neither you or your other single sister, I can see your sister's point. Which sister is she supposed to choose? The one who got married and didn't ask her to be her maid-of-honor. Or you, to rub it into the face of the married sister who didn't choose her?
Thinking outloud, maybe she was rationalizing to herself why your other sister didn't ask her to be her maid-of-honor. I think it's more about HER than about you.
I have two sisters, only one of which is still single. She was our other sister's maid-of-honor, and the sister that was newly married was my matron-of-honor only five months later, and so that leaves me "in line" to be my single sister's matron-of-honor (unless she chooses someone else). But, I'm not really counting on the job, when the time comes, because I'm living in another state, and they live within a mile of each other. I've "teased" them both that I moved out of state to shirk my responsibility, so that the bossiest sister (the one whose maid of honor, I was) could do the job twice (she's very organized, and really did make an awesome matron-of-honor).