These conflicts come along with being married, unfortunately.
In the future, when it is your family's turn for the holidays, I would have your DH mention to your MIL ahead of time (like, say, end of November), "Hey, when do you think you might have the memorial service? Because we are going to be out of town by such-and-such a date and we really need to do it before then." If MIL is not unreasonable, she would honor that, and if she is refuses and does what she wants anyway, you don't go.
For this year, though, I think you should go. Try to look at it from a different point of view. You're upset because you won't see your family for an extra day, but your DH will never see his father again. This service is probably very important to him. And there's no reason to assume your MIL planned the service this way just to mess with you. She probably has a lot of other people's schedules to consider, and she may have thought that as long as she booked it for a few days before Christmas, it wouldn't matter. Unless your MIL is regularly manipulative, I think it's a stretch to assume that she did this on purpose.
Are you going to be with your family Christmas Eve AND Christmas day? In that case, three days with them (through Sunday Tuesday) seems like more than enough time. I mean, I get that you don't see your family that often and you hardly get to see your brother, but I still think three days is pretty darn good. Plus, if you're feeling sour about the whole thing, I would tell your DH that, because your giving up a day for the memorial service, that you would like to make a special trip out to visit your parents for another weekend in January or February. I know you won't get to see your brother then, but at least you'll get to visit with other family members, and it might make you feel better about things.