Why should she pay 1/2 of the rental car? That makes absolutely no sense. 3 people are renting the car, so 3 should split the cost. For the hotel room, there might be an argument if you take about beds (2 people in 1 bed, and 1 in the other) but still, 1/3 is reasonable. Do you really think your sister should pay for half of the meals and things.
As another poster stated, make sure you work out the details BEFORE. It sounds like your husband has some issues (you commented that you think it should be 1/3, I don't think your sister thinks it should be more than 1/3 so that only leaves your husband) and obviously need to be part of this discussion.