Relational as in "kinship"? What has that got to do with Shihan which is a honorific title.
In Japan the relationship one has with another person is important. I understand that someone cannot say to be shihan: others refer to you as such.
From the thread you referred to I understand (accoring to Jun) that the O means grand, not older.
If you look at Morihei Ueshiba's kyoju dairi certificate you will see that it's signed "Takeda Sokaku Dai-Sensei". The "dai" is the same kanji used by Kisshomaru Ueshiba for the "O" in "O-Sensei", but with a different reading.
To me this makes sense: sensei (teacher: self proclaimed), shihan (example:according to others), O Sensei (grandmaster: according to others)