This episode was sub-par in every possible way.
The script was confusing and at some points absurd. It think one major flaw was the way McGee's grandmother was written into the story. Her personality ranges from every from loving to cold to passionate to apathetic to rebellious to docile, for no apparent reason. You could tell Lily Tomlin wasn't able to figure the part out at all, because she just seemed bland throughout the episode.
It all just seemed to snowball from there. McGee's parts, especially in dialogues with his grandmother, seemed lackluster. Even Gibbs, whose character isn't exactly excitable, seemed even more subdued than usual.
I think part of the reason this show was so lackluster was that, as has been mentioned, we've gone through this before. I seem to remember another case in which a male agent has had a female matriarch mixed up in a murder, been afraid to push her, but was forced to by his superior (If this doesn't ring a bell, check the Mothers Day synopsis).
And speaking of repetition, McGee is going to get in touch and work things out with his estranged father. Almost like the Gibbs made up with his father after a decade, or Tony with his dad, or Ziva with hers. Great, the team is 4/4 with estranged dads. And whats even weirder is that all of their mothers are gone.
It's just getting old, there seems to be no creative spark. I know it sounds a bit harsh, but maybe they could use a shake-up in their writing staff.