I've had this bug, which it is, too while playing skyrim a couple years ago. I had to reload an older save but it seems they have fixed it so maybe you need to update your game?
Maybe siding with the empire or at least get to the beginning of their jagged crown request will help you out and then try getting the stormcloak quest.(I havent tried this myself)
Check out the bugs section on this page and the discussion on this page.
That should help you out as much as possible and maybe you can add your findings afterwards.
Hope this helps and good luck!
Frank Voors
