I have a few major problems with my dog rig that I am not able to get right. The back-leg.002 bone does not rotate as it should (it goes only to a certain degree) if I would like the dog to get in a sitting pose. This is possibly one of the reasons that when I try this pose, the mesh of the legs collapse on to itself. I also added IK bones on the back bones, but I am not able to keep the shapes sitting in place.
https://drive.google.com/open?id=10KhKEfXsGzTg_vV4Mv8diULpsWWGhyIK
Thanks ! Sorry for the late reply, but I will get back to the exercise with your advice.