Wow...... Given the potential for ugly drama, my feeling is that you should have the party & just have Dad's family over. The problem is that if anybody ever slips up and mentions it, later on, then your Mom and sister will know they were excluded. Can you handle the potential fallout? Is it worth the risk?
If you think you can get away with it, though, I'd say go for it. (I don't normally advocate such things but in this case I think you, J, your Dad, and your Dad's people have a right to enjoy a happy, fun, no-hassle event, and bond with each other without chaos and drama.)
As for your sister, the only thing I can suggest is maybe editing out the party stuff in your post above and printing out the rest. Ask your sister to please, please just read it, think about it, and maybe you two could talk it out later --- sort of a no-pressure "can you just hear me out?" thing, except she can read it on her own time and it's less confrontational.
You made many great points in your post, and maybe she'll think it over when her defensiveness is at low ebb... I wonder if she thinks your life is just awesome & struggle-free, and so why not do the same in her own life? Maybe she needs a reality check...
