Clone yourself? Are you serious you're born with her if you don't have one by now then lose hope.
How To Get A Twin Sister?
Well, if you haven't a twin sister yet, then you won't, because a twin sister/brother of yours has to look identical and they have to be out of the mother in around 5 Minutes, to have a twin, otherwise it's not a twin.
If you were born already its kind of late to get a twin sister.