Grrrrrrrrrr this makes me so mad for you!
I think given her 'history' of being toxic that it would be appropriate to only allow supervised visits, or if that's not cutting it, no visits at all.
My family respects my choices (though they don't agree with all of them) but I still don't leave DS alone with them because they don't fully UNDERSTAND my dietary choices. Until they can understand them (meaning understand what 'no meat' & 'no eggs' etc. mean, not necessarily the reasons behind them), they won't be watching DS alone -- wouldn't even go to the bathroom if there's food around. (I should also add there are other reasons for not leaving him with them, not just diet).
But I am incredibly strict with diet. I don't believe it's OK for DS to have "just a little" whatever -- to me, the things we avoid are toxic, I'm not willing to have them put 'just a few' toxins in his body, anymore than I'd allow them to give him a sip of vodka or something. I don't understand the impulse to feed kids foods other than what the parent has OK'ed, unless there is a severe lack of suitable foods & the kid would go hungry or something. *sigh* I'm getting more & more frustrated just typing this response...
Co-sleeping is really wonderful when your child actually SLEEPS!!