Although I don’t agree with the tone, there is one thing that I agree with Shayna on … if I may play the devil’s advocate for just a moment …
Weddings are a social function not just for the bridge and groom, but for the parents as well. They’re celebrating the daughter or son finding a partner … and welcoming this new member into the family.
Maybe it’s just where I’m from, but in the midwest it’s a social courtesy to return an invitation … and where my father attended the wedding of his co-worker’s children and his “buddy’s” children, he then wanted to invite those people to MY reception (as I said in a previous post, I eloped … but my parents paid for a beautiful reception).
Honestly, I didn’t think of it as a burden and I didn’t say “I don’t even KNOW these people …” … I thought of it as an honor that these people thought enough of my dad to want to attend. Plus, I had to realize that there were x number of people there to visit with ME, see ME, socialize with ME … I thought it was reasonable for my parents to want to invite people THEY enjoyed hanging out with … if that makes any sense at all.
Having said that though … my father didn’t make me sacrifice MY friends to invite HIS people. This is the huge (Grand Canyon sized) different in our situations. Where I was able to be gracious and welcome these people at my reception, your sister will be viewing each one of these guests with resentment because they took the place of someone she wanted to include.
Also, my parents weren’t divorced … so I only had ONE set of nutballs to deal with … okay, if you count my in-laws, two sets of nutballs! haha 
I can only imagine the amount of stress there must be on your sister just trying to make all these parties happy. The whole situation with your mom’s friends being the ex-wives of your father’s friends … whew … this just seems like a pile of oily rags just WAITING for a match to walk by. haha I know it’s a small comfort, but she’s so incredibly lucky to have YOU helping her through this.
Here’s my thought … before you do anything else (cancelling flowers, limos, etc.), find out exactly how many people she feels are being excluded … if it’s only ten or twenty, call the caterer … ask how much it would be to add those additional people … if the price is too high, ask them if they would consider dropping an hor d’ourve, changing the main course (beef to chicken … chicken to pasta?), basically, bargin with them to get those extra heads in.
Should you have to do this? No. Does someone need to? Unfortunately, yes. In a perfect world, your dad would be the one to call and make these arrangements … then maybe he would realize what a burden it is to change things in the last 30 days. Will that happen? If he’s as stubborn as my mom was (and it sounds like he is), you’ve got a better chance of ice skating in hell. haha
You’re great … seriously … everything you’ve offered to do (helping with the costs, talking to your dad …) is very generous. Keep a calm head … go give your sister a big hug … and see what you can arrange this weekend.
Best wishes!!!