I have tested this fairly extensively, with all 5 pieces on body to avoid bar swap weirdness, with 6 pieces just to eliminate incorrect set application, etc.
With the full set on body and Mother's Sorrow on jewelry and weapon (second set does not matter) I have ~44k magicka with no pets summoned.
If I summon either my Twilight or Volatile Familiar that should go up to ~47k as per the 5-piece.
However, it instead goes DOWN to ~43k. Further pet damage seems to be adversely affected as well, and damage tooltips are all lower.