Oh, dear. It seems both sides have made a bit of a mess of this: you for not cancelling your agreement to be bridesmaids when you were pregnant and she for not doing it, either.
Having new mothers as bridesmaids is a silly thing to do, IMO. Their minds are not on "the job", and rightly so! I wouldn't dream of agreeing to be "on call" as a bridesmaid when I might have to see to my DC, the more so because of breastfeeding!
Having said that, the additional bombshell you mentioned - having new mothers as bridesmaids and not wanting their babies anywhere near the party - is just ridiculous and unkind. Even if she is doing it out of a sense of misplaced "fairness" to other people whose sprogs she banned, brides make "exceptions" all the time, and she is putting you in an impossible position which you are going to resent.
When is the wedding? It sounds like it is soon, in which case it would probably be unfair to cancel. However, you and the other bridesmaid will have to explain to her about breastfeeding, give her a rough schedule, and say you will have to dash off (AND that the baby may be seen at the party, as grannies come in to get you). It can't be unacceptable to the bride; it just can't. This is a human being who needs its food, who hasn't developed the stamina to "last", and who will get hysterical (not nice to the babysitter either, by the way).
If she fails to see reason after a discussion, if/when she has kids, she will no doubt be ashamed of herself!