Hmm, I think I'd differ on this to most other posters. While I love children at weddings, (had 14 at mine!), I do think it's up to the bride to decide what style wedding she wants, and to invite who she wants.
If it were just a friend, I'd send a nice letter saying just that you cant' attend, I dont know that I'd bother to say why. Getting married is stressful enough, and I'd not want to give a bride grief befoer the event, however much I felt she was in the 'wrong'.
But this is close family. YOu need to think whether or not you want to cause this sort of strife in your family, this early in their marriage? Do you want a relationship wiht them? Are you prepared to have this hang over you in the future?
Personally, as its only 4 hrs away, I'd arrange to bring someone with me, a good friend or relative who's not going to the wedding. Have them watch ds through the service, then meet up, nurse him, play with him, (this bit is always long and boring anyway with photographers etc!). Then park the babe with friend in a hotel room, and go enjoy the party. Have a cellphone with you, so that you can go up whenever you're needed. Pop out regularly to see baby anyway.
That's what I did for a friend's wedding when dd was a baby. In fact, other people did bring their children, but the invitation did not include them, and I felt it was rude to just turn up with an uninvited child. Dd was fine, I nursed her regularly in the foyer of the hotel, and in the end, brought her in in her sling. We have some great pics of me and dh dancing with the baby smushed in the middle in the sling!
I wouldnt cause strife over something like this. Just accept that SIL is different to you and who knows, in the future when she has kids, she'll probably look back and understand your point of view. But in the meantime, let her have her big day the way she wants it. It is the couple's day, after all, isn't it?