@Newmum201888888 I assume you are the same person as @Lisburnlane219 ?
Assuming you are, I recognise the fact that you didn't post in AIBU, so the responses will be a bit more measured.
Have you Googled 'babies choking on rusk'? I haven't, not least as Google didn't exist when mine were babies. But if you have, and rusk is a proven risk factor, you might have good reason to ask your MIL not to feed your baby on rusk. I still cut up grapes for my DC (who are all 15 plus), so am not averse to risk factors. I have one who is severely nut allergic, too, so also know about avoiding allergens.
FWIW (which is not much, I'll grant you), DC1's nut allergy became evident on my own scrupulous maternal watch. I gave him a piece of nutty bread, because I love the stuff. I had absolutely no idea that it would produce a reaction in him, as we had no family history of nut allergy. Not quite sure what this shows, other than that you can never rule anything out, however careful you are, and that there is more to making a child feel loved and safe than whether or not they are fed rusks at 5 months.
If it's just about you thinking you wanted to be the first to do X, Y or Z, then you either need to get over yourself, or you need to look after your own baby, all the time. The latter would mean that you could make sure your baby is only ever fed the way you want it to happen.