Erica I'm so sorry that you're going through this its super shitty and I can totally relate to you. My FI's dad and his girlfriend are the same. He actually had the nerve to tell us we were spending too much money, that we were not focusing on the marriage and only on the wedding, and that we were making this into an "event". Well no shit! it is an event.
Honestly I think that its futile to try and understand why people are the way that they are. If you did understand it then you would be more like them, and you're not. You're a caring, loving person who wouldn't miss out on her child's special day, or any other day for that matter. So you're probably never going to really understand why his parents are the way they are...and that's a good thing. Its a tough time for him, and even if its something that he expected or is used to (that's what my FI says), its still not OK. Be there to support him and encourage him in his decisions. If his family wanted to be a part of your wedding then they would make the effort, but as such, there is no use trying to change your wedding to accomodate them, in the end they'll be equally as heartless. Remember all the people in your lives that are super excited for you guys, and for your wedding. Try to focus on the good and not the bad. And remember that when you do have your own family your FI will be an amazing father because he will want his kids to have everything that he didn't.
k that's my rant for the day...lol! sorry my message was so long