"Mum. DB is an alcoholic. One that ruins the day and takes pleasure from it. I dont wish to offend you but I would like a nice, normal Xmas just once. Here are the options..."
Let her make her own, adult, choices.
...your mum absolutely knows your brother is a problem, as you stated "mum got angry saying you only want to do that because of DB"
She KNOWS he's a problem, but is protecting him.
You don't have too. Hes even admitted to spoiling it on purpose. It might cause problems, but you don't have to be a scapegoat or out up with his behaviour and ruin your happy day.
Alternatively, if you really are too scared to rock the boat, develop covid symptoms you need to isolate with a week or so before Xmas day. You'll have to miss this year, then next year its just a simple "no, we had Xmas alone last year and the difference was outstanding."
However I don't recommend option 2, as you still have to face the music at some point!
Let us know what you do, op!