First and foremost, she is your daughter. Whatever you say or do goes, nobody else. Period.
Secondly, if you can't afford it, don't do it. Trust me, two kids and thousands of dollars worth of debt later I wish someone would have pulled me aside and stopped me.
Is there a compromise that can be reached? Is you husband able to attend and have the two you stay at home where your daughter is most comfortable?
I'm sorry that your inlaws are acting in that way. I am an overprotective mother, but there's really nothing wrong with it. Our job as mothers are to help ensure our babies' health and happiness since they are unable to do it themselves. HTH.