Wish for a child - although there is no evidence that you can increase your odds for twins this way, it is possible to wish for a child at a wishing well if you have The Sims 4 Romantic Garden stuff pack installed.The Elixir will increase the chances of conceiving twins or triplets for a limited time when drunk. The Elixir of Fertility - This is a craftable herbal remedy that can be crafted if your sim has maxed out the Herbalism Skill (available with The Sims 4 Outdoor Retreat pack).It will also increase the odds of conceiving twins or triplets when trying for a baby. This massage will get Sims in a flirty mood, increasing the chances of romantic interactions. Fertility Massage - this is a Spa treatment you can request if you have The Sims 4 Spa Day installed.On A Dark Ley Line lot trait - this acts in a similar fashion to the above lot trait but is geared to producing vampire twins or triplets and came with The Sims 4 Vampires pack.This lot trait massively increases the odds for twins, triplets are still an uncommon result, however.
On Ley Line lot trait - this is a lot trait that arrived with The Sims 4 City Living and can be applied through the build mode system.Other than buying the Fertile reward trait and using cheats, there are numerous in-game mechanics that can increase your odds for children these are:
