It always makes my day when I log on and see a new Post-Hogwarts story. It is my absolute favorite genre!
In general, stories where Harry has a long-lost sibling tend to be a tad cliché, normally because it's such a hard type of story to pull off. What you're going to need to do to make this story great is become obsessive about every little detail about how Holly got to where she is. Then again, a cliché that is well pulled off often makes for the best kind of story, just because the writer has to work so hard to make the story original.
Also, I'm curious at what year this is taking place. You say that the story is Post-Hogwarts, but Holly is fourteen, and Harry's parents died when he was only a year and a half old, so Holly couldn't be more than a year younger than Harry for them to have the same parents. Or is she older? Or do you have a little trick up your sleeve for explaining this?
Sadly, I also have to agree that Holly comes off as a Mary Sue. Special powers for no apparent reason (and sometimes even with good reason) do nothing to make a character more interesting. Part of what made Harry such a great character was that, despite all the situations he found himself in, he really was a very ordinary child. He was not particularly powerful as a wizard, intelligent, or even courageous, despite the fact that his convictions always led him to do the right thing (even if everyone else disagreed with him). Make Holly a real person above all, and make sure to give her some believable flaws.
Your story leaves me wondering about all kinds of things. But, I suppose you will be revealing more as the story goes on. Just keep your writing strong so it can continue to be a great story!
Are you a member of the Beta Boards? You really should join. The people there are all obsessive, not just about Harry Potter, but writing in general. We're a community, doing everything conceivably possible to make on another the best writers we possibly can.
Plus, you can get Sorted into a House, and then earn House Points!
Hmmm.......This is a very cliched idea, Harry having a sister.
Your writing is very good, you have talent.
However, Holly came off as a bit of a Mary-Sue and like I said it is somewhat of a cliche in fandom to give Harry a sister.
But the patronous/Samantha idea is a quite unique one and it intrigues me.
This was a good beginning; just be careful of cliches and Holly venturing to far into Mary-Sue-dom.
I hope I didn't offend you, but I always try to be honest about my feelings for the story.
Good. I'll be watching for story updates.
Wes....who is a girl!
I like OC stories, and I like next-gen stories. I think you should consider all of the special attributes and abilities you are so casually bestowing upon your characters, however. "Harry's long lost sister" stories are very hard to pull off (in fact, I've never seen one written credibly). I think your characters are lively and you add a lot of imaginative details. However, your story would be much stronger if you find a beta who can help you get rid of Mary Sue qualities, which I'm afraid are rather abundant here.