I think this sort of thing can be very damaging in a marriage, when one person becomes slim and healthy and the other doesn’t, and resents them for it. I very much doubt this is really about the mil. It’s more about how the op feels about herself.
it can also then cause additional issues ie insecurity , will he look elsewhere, and hearing even basic compliments to him on it will shine a light on the fact she’s not lost weight. Jealousy snd resentment can make everything feel very different,I’ve a friend who when I lost the weight if anyone commented would sigh loudly and look away, in her head it was a barb at her, it wasn’t.
the fact of the matter is, irrelevant of what her husband eats, he’s eating in a defecit and has lost the weight, the op even if she eats healthy is eating too much of it, thus not losing weight, neither are healthy,
if the husbands diet is as bad as she says, and I’m not convinced it is, he likely eats healthy most days, and there is nothing wrong with a sandwich for lunch, but if it is, yes he needs to est nutrionally better. But she also has to improve her diet, not nutrionally but less.
and buying the kids chocolate croissant or whatever once a week is absolutely fine. The ops making out like he’s gorging on junk food every day and piling it into his kids, and again, I doubt this is the reality
the reality is only she can fix this, it’s her choice not to use the jabs, that’s fine, and valid, it’s a personal decision, but it does mean if she wishes to lose weight she needs to eat a lot less. The gym won’t do it.