just had a thought if you created the dogs in the CAS the game automatically makes them littermates which is the same as siblings , you could try adopt another dog from the pet adoption or go to edit town create another sim and pet add them to your household then move the sim out so you have control of the other pet
I do have a dog house and actually to ensure that I didn't lose the dog I really wanted, I used a cheat and went into Edit Sim, saved the dog to the bin, and then as you said, I merged him into my household as a new dog with zero relation so it should work. The dogs aren't best friends yet and there isn't a romantic option still. Do they have be best friends until the option is available?
what andro said build up the relationship ( just like sims the relationship has to be a certain level to woohoo/try for babies ) don't forget to every so often while building up the relationship to click on the dog house once the try for shows up if succesful you should hear the lullaby chime
My pets are littermates even though i put one in different houses
and then merged them
