why did morticia never meet the conjoined twins before??

I always thought it was because the twins were the reason Gomez and Fester had their horrible fight, driving Fester to leave home for 25 years ago.

When they're in the vault together watching home movies, Gomez apologizes for hitting on the twins -- the objects of Fester's affections.

So, knowing that, it's pretty reasonable that Gomez would not want to see them again --or introduce them to his family-- or interact with them because they were the cause of this huge fight.

Add to that... The twins have been an insane asylum the entire time. Not so easy to get out and make visits.

added to the answer below, it was 25 years ago and gomez and morticia probably have been married for around 15 years if you take into account the kids ages and a little time to be by themselves. makes sense to me