will alien 5 and Prometheus be canon ?

Prometheus is Canon and I am sure Alien 5 will be too. Seems like Fox is rebooting the series much like what was done for the X-Men series, it effectively removed some of the crappier movies out of canon and resetting the timelines. 

They'll be canon until they get rebooted in another 20 or 30 years.

In my opinion, canon doesn't really matter as much as whether the movie is good or not.