I will add that almost all the premade Human Sulani Households start out MerWoke… So, if you’ve added a Keahloha or that other household from the trailer to a played household you can make a household merwoke unintentionally.
The MerWoke is a hidden trait you don’t see similar to traits that decide if a sim is Right Handed or Left Handed/ whether they like Fruit Cake or Not.
Edit: To be fair… I’m not entirely sure how the game handles everything, but I suspect that it’s most likely a hidden trait that flags the household as aware of the existence of merfolk… I’m not 100% certain though.