It looks like that "Twins" sequel is still on the way

"Arnold Schwarzenegger claims that he made the most money he has ever made from Twins, which is part of the reason that he has wanted to make Triplets for so long."

Ah, well that explains it. But the Arnold of today isn't the same as the Arnold of 1988. 

movieweb.com/triplets-movie-twins-2-production-start-date/