ok when I battled cythia on luconosa town the second time I battled her I won again after I won she said something about coming to sinnoh can I really go from unova to sinnoh?
It's impossible to go to Sinnoh in Black/White. You can battle her as many times as you want though in her villa in Undella Town
That's just a thing she likes to say, being from Sinnoh. You cannot actually move between regions in B/W, so ignore it. We can only hope that it'll be possible in future games.