Carrie ann is right. She needs to grow up and get help for her issues. Someone needs to speak up to her or she's gonna continue to wreak havoc on your lives. Nothing is going to stop her unless someone is finally tough on her.

Don't walk on eggshells for your sister. Make your announcement as you would normally and don't hold yourself back because she's self-centered and will throw a fit. If she does you just stand up to her and put her in her place. Seems like just from wht you've said she's been catered to by your family for far too long. Just your considering taking her to "keep her at bay" tells me this is probably a behavior your family has exhibited for a long time - working around her issues. Don't do that anymore!! Especially don't even think about taking her on your own family trip. No way.