I don't know that child #4 is a twin or even male - I'm just spinning out a wild-ass theory to make all the pieces snap together.
I can't help noticing that "Tate" was the name of a famous victim of a mass killing, not the perpetrator. Another clue?
Hugo is probably Tate's father, since Tate was the only normal one of the bunch. The others were fathered by Frankenbaby in the gimp suit.