Is your c/s booked in on your EDD? Normally they are booked early enough that it is unlikely that you will go into labour before then.

Personally I think it would be unfair to ask your husband to miss his sister's wedding. You would have to be very unlucky to go into labour in the short time he is away. I understand how you are feeling, I have told DH that is not allowed to go on any more work trips after next week, but a wedding is different. I would be either going with him, or as Rommy'sMummy suggested, have him not drink so he can drive back at a moments notice.