@oatcakesandcheeseplease - this is all about them and their feelings about their way of eating. They're projecting it all on to you. It's very similar to how other parents speak to you about your parenting choices - the way you choose to do things is a direct challenge to how they're doing it - and so they're trying to assert that their way is the right/best way.
When my DC were small, it was just as organic foods in supermarkets were gaining a real, mainstream/mass market foothold. And I used to buy a lot of organic food, wherever possible. All the milk and eggs that I bought were (and still are!) organic, along with all the fruit and veg that I could. I even used to buy organic gin - although I didn't let the DC have any of that 
My parents came to stay one weekend, and my mum got really angry about what I was buying. I could have understood if it, if it was criticism about how much I was spending (even though we were able to afford it), but it was all just 'sound and fury' about the ridiculousness of buying organic. I couldn't understand why she would get angry about me doing something that should (in theory at least) be better for my children's health.
I realised some time later that of course she was angry with herself because she didn't do the same when she was bringing me and my brother up. She was angry, in other words, because she saw me as practising 'better' parenting.
I don't know what I'd do in your shoes - I have a similar-ish issue with my SIL, who is an ex-WW leader (but also at least a size 18-20 and constantly searching out no/low fat foods, believes low carbing is a fad, blah blah blah) and will simply not countenance this WOE. However, I don't have an easy relationship with her, so I just go with the flow and eat whatever they are when I'm with them - which means, of course, going out for pizza (in a restaurant that only serves pizza
), a house full of no/low fat food - but interestingly, all manner of 'naughty' foods, like Magnums (always mini, of course, as that's less naughty) and crisps.
If it was my other SIL, I'd have no qualms about tackling things, because I know she'd listen a bit more. We might agree to disagree, but we could at least talk about it.