Hopefully this will be fixed. I'll add this thread to the bug list.
Here's a simple example of this issue with Sims that have only one trait, Good Upbringing:
Normal duration of family event: 60 minutes
The card shows 54 minutes as if GU from both Sims could affect the duration of the event.
Correct time should be 57 minutes as only the triggering's Sim GU is effective.