"I feel depressed because it reminds me yet again of how all the responsibilities of parenting are foisted onto our household because the self-indulgence is so great in the other."
Anna, have you ever thought that if your partner didn't have an ex wife on whom to blame every problem the DSSs have, that blame would go to their friends, the teachers, the TV culture or even their grandparents?
That's what happen when there is no ex-partner on sight. People seems to have an intrinsic need to blame other people for their own problems.
In any case, I doubt she force feed them, therefore the responsibility for eating that much remains on the children and what they are used to... consider the following:
My mother never had biscuits at home because that would make us fat. The survival rate of a full biscuit packet in my house, and those of my sisters, is barely a few hours.
My aunt, in contrast, always had biscuits and other rubbish readily available for her children to take. They were always thinner and healthier than us, and they can bin biscuits because they go off before they can eat them, they always had them they are not as attractive to them as they were to us
Perhaps being so strict about food is the reason they ate so much while away from you? Just something to consider?