I don't see a huge issues with going to a wedding if you feel well. Obviously not everyone is upto it but plenty of people are and you may enjoy it.
I felt great after dd1, only bled about 5 days and also went to a wedding exactly two weeks after her birth, staying from noon until about 9pm in the end! Dd didn't come but slept most of the day with Mil so wouldn't have noticed me being there. Of course I missed her terribly but it was a close family member on my side and it's only one day and weddings are special. I have the rest of my life to spend with dd and felt a few hours apart for such an important day was acceptable.
However I definitely do not think you should be bridesmaid. It's totally different to being a guest and if you don't realise that then maybe that is why the bridge and groom have suggest a babysitter!
As a guest you can come and go and your baby can fit in pretty easily. As a member of the wedding party, you will be expected to be on duty until pretty much the evening. It's not fair to take on the role and then not full fill it. This means helping the bride get ready, having photos in the house or hotel, being up front at church, sticking around for the entire set of photos as opposed to just friends and guest ones. Also you'll be on the top table all throughout the reception. Who will mind the baby during the day if you both have key roles?
Actually I think that's part of the issue. You and your dh both have major roles in the wedding but neither is likely to give 100% with a baby there plus I understand if they dont want a baby on a pram on the top table! It's different when its your own but it's their day and understandably they'll want their key figures to help them out. Weddings are stressful.
Your dh should def go and don't think there's any issue with him being best man for his brother. I really think you should go as the guest then you can slip away early if needs be or nip out to settle and feed baby.
At least one of you can be there for them properly on the day. Of course you may be fine and enjoy it but I think it's very unfair on the bride and groom to pull out as a bridesmaid last minute.
Also if you are not feeling upto it at all, at least one of you will be there.