You can't click "Make Sims Fight." But you can engineer the conditions where conflict erupts organically—just like the Pleasant family's scripted tension between Daniel (Knowledge) and Mary-Sue (Popularity), or Lilith (Fortune) vs. Angela (Family).
Daniel (Knowledge, Serious, Neat) + Mary-Sue (Popularity, Outgoing, Playful) = guaranteed clash. Lilith (Fortune, Active, Grouchy) vs. Angela (Family, Nice, Neat) = sibling rivalry. Build that DNA into your CAS and watch the soap opera write itself.
Don't force it. Design the powder keg. Light the match. Observe.