DuBarry 5020 Fitting Help

I'm currently working on a muslin for DuBarry 5020.  I think I have most of the fitting issues that need to addressed identified but I need some help on how to go about fixing one of them.  The problem is that when I lift my arm, the entire blouse comes up with it.  I need add a bit of width to the lower part of the sleeve but I don't know if that will have any effect on this particular problem.  If you can spare a few minutes to help me out, I'd greatly appreciate it!  There are more pictures and a better explanation on my blog, Sew June!  TiA!