Look at it like this. What if you had adopted her. You would not know any of the experiences of being pregnant or giving birth. If there is a chance these people might met her Mom, play up the aunt thing.. I would not tell anyone of your trans past, it isn't any of their business.
I have done the whole been a Mother, because people who knew me before, knew I had four grown children. They would forget sometimes and we would be talking about be pregnant and giving birth. I just used the experiences of my two ex wives to fill in the blanks. Did I lie, or did just CYA? If they think you are her Mom, that is their observation. Just go with it.
Why would you want to raise up a problems you might get if you were open about your true relationship to daughter. You are her parent, her your child. If people assume that you are her Mom, then why blow their little image,