Roof Rafter/Beam Joint pulling apart and sagging
It looks as though this thing was built out of scraps pieced together.  There is just no way this is ever going to be sound the way it is.  

I think your best course of action is to build some temporary supports to keep the roof from collapsing, pull out the substandard rafters and rebuild using correctly sized lumber, supported properly.

If you're in doubt about how to do this, you may want to hire a professional.  I wish there was some quick and east fix but unfortunately I just don't see that in this case.