I don't think any guest in their right mind would make an issue if your sister was the only child invited. I personally think it's more wrong for you to not invite her. It is not unreasonable to ask that there not be kids there. To expect your sister to be excluded because of it is too much.
"I came here tonight because when you realize you want to spend the rest of your life with somebody, you want the rest of your life to start as soon as possible." - Billy Crystal, When Harry met Sally
I too think this may be a fake poster. I mean come on.....she is your SISTER!!! Wow.....if this is a true post you really need a reality check. I think who ever said you are the one who loves drama, and not your mom is right.
And this is not a CRISIS.......she is your sister. Get over yourself.