ARGH!

I'm rather frustrated right now. Here's the backstory:
DH's sister is pregnant and is due in one month. We've been asking for MONTHS when the christening would be. Since it seemed that SIL and BIL would plan the christening for when we would be up north (they live in New England) this summer, I emailed SIL as soon as I had two dates (one Saturday and a later weekend) on which we had plans. I gave her practically the entire month of July, mentioning that my parents will be celebrating a milestone anniversary this July, but we'd schedule that celebration around the christening.

Fast forward to 2-3 weeks ago:
SIL called the house as I was running out to pick up the kids. I let the machine pick up. DH spoke to her about a day later and told me that she was hopping mad. It turned out that she asked her DH's family when they'd be coming in for the christening, and mentioned that they would not, under any circumstances, consider doing it the weekend in July when we'd be away. The family could work it out and come up with dates. Well, they did. They came up with the weekend that we're going to be away with my family. And not only that, but they all booked their airline tickets as well!! SIL told them that that was completely unacceptable, since we had already told them the date would not work, and the family had been told that. She refused to have the christening without her brother present. BIL didn't understand what the problem was, after all, we are ONLY going to an amusement park for the weekend, and we can easily change our plans. Not quite. We are going as part of a group, and that is the weekend. The tickets we purchase will not be good for other days. SIL told this to her DH (that we are going on a corporate-sponsored trip, and the dates cannot be changed for the thousands of people attending) and that his family will have to change their plans. Hey, they didn't follow the rules.

Fast forward to last night:
DH had called his father and grandmother. After the call I asked him what the situation was regarding the christening. So he told me that "We can't go." His family refuses to change their plans, since BIL's father's birthday is apparently around that same time, so they are making some sort of christening/birthday weekend thing. Which would be fine and dandy, but no one has told us that Pop is turning some special age, which would really matter. Most adults I know don't care when or if their birthday is celebrated, so does Pop's birthday really matter for that weekend, or could they move it one weekend either way?? I dunno. Apparently SIL was also told that it would cost $1600 to change the airline tickets. Quite honestly, with what I understand SIL and BIL bring home, they could probably afford to offer BIL's family the $$ to change their tickets. Heck, I'm so annoyed, I'm even considering offering up the $.

WTF?!

Now I have to decide what to say to DH about what we as a family should do. We've all been looking forward to this long weekend. We get afternoon admission tickets for Fri, then all day Sat tickets and a big BBQ, then tickets good for any day Sunday to the end of the amusement park's season. Our plan was to get there Fri afternoon, go to the park Fri, Sat and Sun, and then head back to my parents' house on Monday. We'd be able to take a leisurely, scenic and educational drive back, giving our oldest something to talk about at school this fall. Do I tell DH to go alone to the christening? Do we give up the weekend with my parents, brother, SIL and their kids, and go to the christening? Do we attempt to do both, especially considering it is about a five hour drive between locations? Or do we boycott the christening and enjoy the weekend we originally planned? Does anyone else see other options? Anything I haven't considered? If you have any ideas, please comment.