At this stage you have no idea what kind of birth you will have. Baby could come early/late, you may have an easy natural birth or you might end up with an emergency C-section. You literally have no idea, until labour starts!
The fact that your sister is against your newborn baby even being at the same venue, in a hotel room because she thinks the baby will take attention away from her is immature, mean and utterly pathetic. She's expecting you to travel 20 minutes to the nearest hotel, so you can feed your baby and come back again!!! I am making a massive assumption here that your sister doesn't have children!!!!
At this point you would be better to bow out. Tell your sister that you will have a newborn baby, will be breastfeeding, and her requirements aren't going to work. You won't be a MOH or attending her wedding, as you can't travel to another hotel, every time your baby needs feeding.
Even if your husband gave up his groomsman role, unless your baby takes to a bottle (mine son refused bottles when I breastfed), he won't be able to feed the baby. You still may have to travel 20 minutes to another hotel, to see your baby and feed them!!
Do your parents know the stipulations that your sister has put upon you? I don't have siblings, but if I did, and my Mum knew my sister was wanting such ridiculous rules, she'd be given a metaphoric kick up the arse by my Mum!!