Everyone could have handled this better. Your sister was out of line, as you had asked her in advance to be more considerate. Your wife should not have yelled – that did not help the situation. Your parents were out of line for not speaking to you, and for not defending your position with your sister.
Lesson learned – no more vacations with your family in the same vehicle. For 28, your sister sounds like she needs to grow up. A LOT. That said, a baby should be trained to sleep through noise. Some cannot be, however, and your sister should have been mroe sensitive. In fact, because she thoughtlessly woke him, she should have had to be up with him for the next three hours.