I know the Watermelon, Last Chance Saloon, Angels are all a series with sisters.

I just purchased Anybody Out There? Do I need to read the others first? Will it mess me up if I go ahead on this one?

I'll be honest, it's been a while since I've read anything by Marian Keyes but it seems to me that you'd be okay to read them out of order. Personally, I read Rachel's Holiday first and always recommend that one first because I loved it the most. But, I don't recall there being any specific reason why you couldn't read them out of order.

IS ANYBODY OUT THERE IS COMPLETELY UNRELATED AND NOT USUAL KEYES, HER BOOKS ARE USUALLY VERY FUNNY BUT THIS ONE IS ISN'T

You don't have to read any of them in order. Not even Watermelon, Last Chance Saloon, or Angels. I have read Angels but not the other two. I also started with Rachel's Holiday and just read them as I as so desire.

I've read them all, you dont have to read them in order.  Watermelon was my FAVORITE.