After reading your post, I initiated the Safari app, went to Yahoo's mobile site and successfully logged into to my Yahoo! mail and I am using my 4S with iOS 6. Try logging in via a computer and see if you're successful. If you aren't then the problem may be with your login credentials. If you are then I would suggest you download Chrome for the iPhone and try logging in with it. If you can with Chrome and still cannot with Safari then I suggest you reset & restore your device as new and try again.
yeah i can sign i now but it keeps showing me the same emails everytime even after i delete them it never brings up the new ones it shows me i have new ones but never loads the new ones just the others everytime..