You need to sell to the Smoth family.
An easy way to get this is opening up these houses and immediately selling them:
Connoisseur's House, Home and Car, House that is Hiding Something or Boring House.
Make sure to have a minimum of 2 Bedrooms
Make sure to have a minimum of 2 Bathrooms
Make sure to have a Living Room.
Make sure to have a Family Room.
Make sure to have Book shelves.
Make sure to have large Beds.
Make sure to have a well stocked kitchen.
I would recommend to have 3 bedrooms as they do want to have a family.
Essentially try to make sure the house is a nice family house.
Living room requirements:
Sofa
Coffee Table or Table
Family room requirements:
Sofa
Coffee Table
TV
Carpet
Toy
Bookcase OR Picture
Kitchen Cupboard OR TV Cabinet