I'm thinking of taking advantage of being the favourite of my (future) MIL and not standing up for the other DIL
What do you mean when you say taking advantage?
SIL died last year. I think MIL might be looking for someone else to be her favorite.
That should not be in your mind. A family member has passed and you should just be there for the family instead of trying to be a favorite.
My mother wants to be the favorite, so what? I will ignore the other SIL because she had a better MIL. It's a win win situation.
Everyone should be treated equally here. You should show all of your future family members the same amount of respect.