Jane, you're human, which means you're not perfect. You do absolutely everything for those buns, and they know it. They know how good their life is. They would be far worse off if they were rehomed.
We all 'slip up' and do things we then regret, but that is because we are human. All we can do is do the best we can at the time, which is exactly what you are doing.
There was no knowing the grass would cause a problem, and given 3 siblings have already died, chances are this is actually nothing to do with you, but more to do with them, although I do appreciate it is easier to blame ourselves than realise it is not within our control.
By blaming yourself you are adding to the suffering you are already struggling with because John-Boy is ill. You're suffering far too much and by placing judgements on the situation (that may not be true), you add more pain, which you really don't need.
I truly hope he makes a speedy recovery.