Well if the house in question cost a million quid, then they must be making some serious money.
Presumably you havent bought your own house because for whatever reason you can't afford to.
Her embarrassment isn't because you rent your home, for example, you could be "between" mansions and waiting for the builders to finish, hence why you are currently renting.
No, her embarrassment is because she thinks you dont earn enough money in comparison to her husbands kids.
No idea why, there are infinite reasons, opportunity, education, luck.
Next time she complains about your "lowly status" just reply
"Well, a childs chances of success are mostly dependent on the quality of parenting they received, clearly you failed as a parent, please dont project your feelings of negativity onto me, have you tried counselling?".