As I said I wasn't able to tell them no, if I did my Dad would no longer be speaking to me as he would see me as the bad daughter, he wouldn't come so I would still have to change everything in order to get him off all the plans etc. So it really was a no win.
What I did do is pretty much what bettymae1121 said. I will not watch them, I will not be responsible for them. I will not give them any money for souveniers except their birthday money (DN 15's birthday is 10 days after we leave so she will get hers early). I changed the ADRs I could and helped my mom find split ones for the other ones. If DN 8 whines she is missing Chef Mickeys etc. well someone else can decide if they will switch with her or not I don't care.
I also am definitely taking the master bedroom with my DH so I have the ability to completely shut them (and anyone else that may get on my nerves) out even when back at the resort.
My mom is very clear that I unhappy about this. We are driving down and its going to be a pain since now we have to entertain an 8 year old the whole time (doing everything I can to make sure she is in my parents car and that I'm in DS 2's where I planned to be in the first place).
I refuse to book them tickets to anything that will cost me money unless the money is physically in hand from sister 3 already (a check doesn't count either becuase I don't trust it not to bounce. Sister 3 is the only one in the family I will not gladly lone money to... and I'm not the only one in the family that feels this way as a few of the others learned the hard way.) So they will get into the halloween party or they won't I don't care.
My Dad seems to think they will get in since he will just give them his ticket and convince my mom to do the same (although I'd be attempting to get her to go since she wants to). However that won't even work since his ticket and the 8 year olds wouldn't be interchangeable.
I also refused to do anything special for the ADRs. So someone else gets to deal with making sure the DN15 (vegatarian) will eat at all the places she is going. (Most I know will be ok, but not sure on 50's prime time) if she won't she can get something else when she leaves.
I did find out we don't have to worry about souvenier money. Sister 1 who also doesn't trust sister 3 with money has kept my neices birthday and christmas money from her and was planning to give it directly to the kids at some point (Sister 3 has been known to take her kids birthday money when an "emergency" like getting an inspection sticker comes up).
I also refuse to take turns staying with DN 8 if she is whining or tired in the rooms or whatever because she can't behave in the parks my mom is already saying that would be one way to handle it but I told her that since She and my father seem to still refer to my husband and I as the kids and don't really take our input on things anyway we obviously aren't mature enough to do that and will just to play in the parks ourselves. They can miss out on stuff I don't care anymore.
This has become much less of a family vacation and much more of us happening to all go to WDW at the same time.