Of course you want your children to be loved and treated equally. To you, they are equal. In the most perfect of worlds, your mother-in-law would be able to accept your sons as her own, even though by a father she never knew, and treat them with as much love and generosity as her by-birth granddaughter. That's the dream, but very seldom the reality.
Try putting yourself into this hypothetical situation. Your oldest has grown up and chosen a wife, who, just like you, already has children. You meet the woman – mmm, okay, you'll work on accepting her even though your "styles" are different. The kids? Well, they're strangers to you and you are a stranger to them. You didn't get to "choose" them as family – that was your son's doing. HE wants to be their dad. Fine. To you they are just kids. You might see the point from the start in "including them in," you might just want your "blood" granddaughter and wish you didn't have to be bothered by someone else's children.
So that's the situation. Here's the big question: Can you force yourself to love those older children? They were presented to you as part of a package deal after their personalities and habits were already formed. You met YOUR granddaughter as an infant, who would know you as family from birth.
Sadly, we don't live in that ideal world, and in the real one, people's feelings are not generally malleable, especially under pressure. I have tried really hard to "love" certain people IN my own family, including my mother. I'm sad to admit, the feelings just aren't there. I can't generate them at will. If my mom were to challenge me, to demand my love, the best I could do is pretend (and sometimes she does, and sometimes I do). And that feels pretty terrible, for me.
The simple fact of the matter is that YOUR feelings, choices and lifestyle are your business, and other people's feelings choices and lifestyles are not. Their feelings choices and lifestyles are their own business. We funny humans often try strategies to influence the feelings and choices of others, some positive, some coercive. The coercive ones (pressure, anger, guilt, shaming) eventually tend to corrode the relationship, and actually block the development of more positive feelings that might otherwise grow over time.
If I were in your position, I would back way off of my expectations and demands. Blocking the junk food – fine, that's a health issue.
But the feelings and corresponding behavior you desire may not be realistic, even though desirable. I'd explain to my sons that this older woman is probably very nice, but just doesn't know how to think of them as family, and that's not something she can help. It's no reflection on who they are or how lovable they are, and if they want to treat her as a special granny, they should be encouraged to do that.
Also stress that the gift giving/receiving is NOT a competition. She simply sees their sister as "her" own family. I'd get them a special toy or two that they can take to G-ma's house when they visit, and bring home again if she's not comfortable finding the space to keep them. I'd also occasionally send a small hostess gift for G-ma along with your children, with a gift card signed by all three of them. Simple things, like a small potted plant, a sun-catcher for her window, a book of puzzles, or things you know she likes. Perhaps even something your children made especially for her. Not every visit, as if you're trying to hammer home a point, but every so often, just as a thoughtful gesture (and one that may gently remind her whenever she sees the gifts that her family now includes other people).
In short, treat her the way you'd like her to treat you and your kids. There are no shortcuts to growing sane and healthy relationships, which may eventually include love and respect.
I sure do wish your family well.