I think you've gotten some great responses. I think, too, that baiting is a possibility. My SS did it to an extreme during the first 2 years of DH's and my marriage and it almost got very ugly when BM threatened to call CPS, but fortunately we got around that when we repeated all the horrific stories he was telling us about her! But at any rate, your SD may just be trying to express her frustration about having you in her life but she doesn't feel able to do it directly, so she does it "through" her mother.
But even if BM is in fact speaking that way about you, breathe deep and take the high road! I would try total non-responses like "Hmmm," or "Oh?" Whether she's baiting or repeating what she really heard, it will get boring for her if you never react. I have a girlfriend from childhood whose parents divorced when we were in high school. Her dad never, ever once said an unpleasant word about her mom. Her mom talked critically about her dad constantly. This did not turn the kids against their dad! They got incredibly angry and frustrated with their mom and are closer to their dad to this day. I know it's not exactly the same situation since there were no stepparents, but even young kids know petty insecurity when they see and hear it.
And, FWIW, if BM is in fact speaking this way about you, just smile a little inner smile with smug satisfaction that she is so insecure that she has to stoop to this. Alternatively, feel a little sorry for her, but I'm not that emotionally healthy myself.
It's easier for me to take the high road if I can gloat just a little while I ride!