I realise I'm a lone voice here, but I think you are being too sensitive about this.
You have been married three times, are smart and well educated, very slim, now earning considerably more than your DSis.
In comparison, your DSis is almost 40, never married, heavier than you, did her degree through the OU and now earns a lot less than you.
I think she probably has her own insecurities, possibly voiced to your DM, and it is about your mother feeling the need to bolster her confidence.
A compliment to your sister is not a criticism of you.
Your description of yourself sounds quite glamorous. Maybe your mum doesn't think you need your confidence boosting in this area. Maybe your sister compares herself to you and considers herself less attractive, fatter, less successful.
You feel petty raising it because it is petty.
If you tell her to stop she will stop, but only because you've asked her to so you won't feel any better about the situation at all.