MIL pressuring us to travel with child?

My Mother-In-Law has been pushing my husband and I (who both work full-time) to allow her to take our 5 year old out of state to visit my BIL and his family for winter vacation when school will be out of session. Being that my BIL is a heavy drinker and my MIL is his “Yes Man” and will overlook his aggressiveness while he drinks, I have said “No” to the arrangement and would rather take time off or put our child in a friend’s care for the week while we work. She cannot understand why we would choose this route and is upset by it. She is now pressing us for answers. How do I politely respond if I am asked?