Installing Minecraft twice on one computer?

I have 2 daughters who play Minecraft. The older one bought an account and we downloaded it to our PC. Now her younger sister wanted an account, so we bought her one as well. The problem is that when she goes to play, even though she signs in with her account, she sees her sister's worlds. Is there a way we can download another copy on our PC, or a way she can have her own account and open worlds to play on?

They are saved on the hard drive, try creating a new account on the pc, not a minecraft one, and keep the save worlds separate etc xD