Delete your remote dev from github page
then apply these commands:
1) git branch -D dev
2) git fetch upstream
3) git checkout master
4) git fetch upstream && git fetch upstream --prune && git rebase upstream/master && git push -f origin master
5) git checkout -b dev
6) git push origin dev
7) git fetch upstream && git fetch upstream --prune && git rebase upstream/dev && 8) git push -f origin dev