I was going to say about Facebook telling you if message has been read. I do think your DH talking to his brother about it is a good idea, but like you say the earlier the better.

I am the same as you I would rather be comfortable and not there than uncomfortable.

I really think it is the strangest thing - in a church normally everyone is forward facing so apart from those sitting next to you (which I assume wouldn't be her older conservative relatives (not that breastfeeding is not conservative) wouldn't even know - and at the reception wouldn't you normally be sat with your family anyway - the request is rude and unreasonable but also there is so little logic behind it.