I like Tabby's solution.
The only thing that I would add is to make sure you talk to BM so that she doesn't bring sushi for 4 and find out that only 2 are eating it. Let her know that you're also making the pasta, and say "Hey, if you'd rather have that instead, don't worry about the sushi." Then it's her choice and you're not overriding her offer.
If she chooses to go with just the pasta, make a large salad so that you and BM can eat on the lighter side.
Then again, WillyNilly has a good idea. Maybe a chicken/rice casserole for DF and DS, something that goes with/complements the sushi.