A simple thank you for your offer to help, BUT I have already taken care of that, or I was actually looking forward to doing that, should suffice.
It's one thing to be a helpful, aunt or grandma, but this sounds more like a power struggle/ control issue by all involved.
It's ok to be direct in your communication, I have found taking more of a professional business tone with one of my over helpful/ controlling sister in law works. I remain polite, I firmly decline, thank them for the offer, and let it go.........
If they have a problem, it's on them, Not You.
We cannot control how others feel.