@mac2cd,
If sisters in law "don't get involved", then your wife shouldn't have to cook. It's an all or nothing proposition. Either she's involved in the planning so she knows what to cook and has input in that. Or she's not involved in the planning and is off the hook when it comes to making food.
Your wife lives in the same house as your sister. She (your wife) has every right to be at home when your future BIL's side is over, and can talk to them directly.