I can't use the name I would have got if I'd been born a girl, because my sister took that name two years later (shame because its a great name)..
I don't see what stopping you. It's not like, at this point there is something, that can define you within a system better, than ID card, which goes with number. With a lot of numbers, in fact. If name's not enough to solidify you in modern society, then it's only superficial identifier, clearly intended for cosmetic purposes. At least, on large scale. To you personally it can mean a lot, and in my opinion, you should have ultimate decision to have whatever name you feel comfortable with. Even, if your sister took it. Everyday life tend to sort out issues that come from such coinsidences.
For me personally, name is important part of identity (if still a part). Every name of every identity I have (not, that they are drastically different, mind me). I think, at least for me, identity isn't that important part of existence. On it's own, at least. But communication is, and if identity stands in a way of that, it should be changed or replaced by other. At least, for as long, as it needed. Some buiding blocks of identity can be persistent, but it's not immutable in it's enirety and name is one of the things, that can be changed to express your evolving, overcoming and compromising to reach out others. I hope, that in future identity would turn into something more plastic and used to self-modification.