For the BP, I wouldn't go. But then my first pregnancy I had m/s for months, so I couldn't even get out of bed. Even if you feel well, you will be in smoky bars and such, so I'd pass.
I'd just tell the bride that although you'd love to be there, for obvious reasons you'll have to skip the bachelorette party. Is there any way you can both go out to dinner or something else...just the two of you? I think that would be a great compromise for not making the party.
I'd totally understand if I were the bride.