It seems like you just began season 5 and, as you should, got confused with the episode "Real Me." Just keep watching and it will be all explained.

So she isn’t really Buffy sister

Rhey created her to life, giving anyone who close to Buffy memories of Dawn. Even same blood of summers, so she was with her father during the divorce in Buffy and mom and everyone minds

Just asking a yes or no

Finish watching the season. It very clearly explains in clear detail where she came from.

So can’t answer few words lol

Looking forward on reboot

Cgi will be greater