Yeah you just need to do grey rock? It's not as complicated as you think it is.
Be pleasant, be kind, offer no details, ask them about themselves, talk in detail about the weather, react to nothing. Become the queen of "mmm, yes, that does sound hard, dear me".
If anyone in the family notices and starts to moan at you about how you're treating auntie/nan, just reply with "oh dear, didn't realise X felt that way, thanks for letting me know", and do nothing, and never mention is again.
Rinse and repeat. Eventually auntie and nan die. Job done, you avoided their nastiness and didn't feed the fire and that's it really.
It sounds harsh etc but really that's the extent of it. The rest of your family doesn't need to be "on your side", or approve of you, or anything. You don't need to placate anyone who wishes you would reveal more to auntie and nan, you do not have to solve that problem. You can just make sympathetic non-committal noises and move on with your life.