Caterpie Family/Weedle Family
Nidoran Family (Male/Female)
Pidgey Family/Sparrow FAmily
Abra/Machop Family (Brains Versus Brawn)
Ledian/Ariados (debatable, but this whole thing is pretty shaky anyway)
Wurmple's split family
Gastrodon (one Pokemon, but still multiple forms)
There are also the version exclusive Pokemon you could argue for, some of which are included in the list. There are a few more you could probably argue, but these are the ones I saw from the pokedex.