If you have, and you accepted it, you will notice that it is a newly created facebook account.
What I have gathered from these things is that it is not really a case of 'hacking' a valid facebook account and taking it over, but more this: Somehow people get a hold of your (or this case your sisters) 'friends' lists. (this is done by the stupid quizes, or games that are on facebook that ask permission to access your friends lists). Once they have your friends, then they copy your profile picture and create a brand new account, use your profile picture, then start going after your friends with new friend requests. Then they request to be friends of all of her friends. (and so on, and so on)
Once a person accepts these fake new friends requests, the scammer can then access their friends lists (and it goes on and on). I think the scam part comes in using messenger (once you are a friend) and try to talk you into investing in this or that, or any number of scams.
The best thing to do, if let your sister know, and have her post something on her page there telling people she has been hacked and to not accept any friend requests made by her. Also, if you receive a fake friend request, you can report it. You can usually tell which one is the real friend (sister) if you look at the profile of the new friend request person, it will say 'new account'. They also won't have much content posted, and few, if any, friends.
If you did not get a recent friends request from your sister, then this doesn't really apply.