This is the scenario I found myself in many years ago.
Now hubby had planned to ask me on a New years eve, however during the planning stage, younger sister’s boyfriend proposed on the bonfire night several weeks earlier.
I had to just take a step back and let her have her glory. It did mean I couldn’t go to any wedding shows (wasn’t bothered) and couldn’t have my mum’s attention for my wedding.
We ended up just sorting out our small wedding ourselves without any help.
She was pretty nasty about it at one point.
I would go ahead and try to be calm and mature about what may happen.