This is hard for me to imagine - to put myself in your place - because we are all Christians in my family, and a baptism would be very important to us. So, regardless of other things going on, everyone would shift their schedule, if need be, and do their best to be there for that hour. That goes for First Communion and Confirmation, also.
It sounds like your family is more mixed. Obviously it is important to you and your son, but your in-laws don't consider it a big deal. That's understandable. If they aren't that spiritual, why should it be? So try to be gracious about it.
I guess what I would suggest is that you simply say to you SIL "Sue, I know it's a busy weekend, but this is really an important day to Johnny, and it's only an hour on Sunday morning, so you'd have the rest of the day for your celebration. I hope that you can find a way to make it. It would mean so much to him to have you there."
Then you have to accept whatever decision is made - without bitterness - and move on. (Because, as Christians, that is what we should do.)
And as long as your son's grandparents are there, I'm sure he will not miss your SIL too much. He'll be thinking about what is happening, rather than who is watching (as it should be). Keep the focus on what it should be, and you will all have a great day!
Tell your son congratulations! And welcome!
Blessings.