Is recurrent simulator training required every 12 months whilst acting as SIC under Part 91?
The only Part 91 recurrent training and testing requirements are in subpart K, so if they don't apply to you then only the general currency requirements in Part 61 would apply.

The SIC currency requirements in 61.55 are (every 12 months):


  (1) Become familiar with the following information for the specific
  type aircraft for which second-in-command privileges are requested—
  
  (i) Operational procedures applicable to the powerplant, equipment,
  and systems.
  
  (ii) Performance specifications and limitations.
  
  (iii) Normal, abnormal, and emergency operating procedures.
  
  (iv) Flight manual.
  
  (v) Placards and markings.
  
  (2) Except as provided in paragraph (g) of this section, performed and
  logged pilot time in the type of aircraft or in a flight simulator
  that represents the type of aircraft for which second-in-command
  privileges are requested, which includes—
  
  (i) Three takeoffs and three landings to a full stop as the sole
  manipulator of the flight controls;
  
  (ii) Engine-out procedures and maneuvering with an engine out while
  executing the duties of pilot in command; and
  
  (iii) Crew resource management training.


Practically speaking, what the FAA requires is only part of the picture because your insurance company will probably mandate certain training and currency requirements that may go well beyond the regulations.