These aren't harmless old grannies knitting acrylic baby blankets by the fireside. Not at all. Check out the description of them:
In the outer room the two women knitted black wool feverishly. People were arriving, and the younger one was walking back and forth introducing them. The old one sat on her chair. Her flat cloth slippers were propped up on a foot-warmer, and a cat reposed on her lap. She wore a starched white affair on her head, had a wart on one cheek, and silver-rimmed spectacles hung on the tip of her nose. She glanced at me above the glasses. The swift and indifferent placidity of that look troubled me. Two youths with foolish and cheery countenances were being piloted over, and she threw at them the same quick glance of unconcerned wisdom. She seemed to know all about them and about me, too. An eerie feeling came over me. She seemed uncanny and fateful. Often far away there I thought of these two, guarding the door of Darkness, knitting black wool as for a warm pall, one introducing, introducing continuously to the unknown, the other scrutinizing the cheery and foolish faces with unconcerned old eyes. Ave! Old knitter of black wool. Morituri te salutant. (1.24)
The thing that strikes us right away is that these ladies seem to represent the Moirae—the ancient Greek personifications of fate. Two of the three Fates spin the life-thread of each human being; the third Fate cuts the thread when the time comes for the man to die. The Fates, being immortals, have foresight and thus can see every man's fate.
Okay, where are we getting all that? Well, look at the way Marlow contrasts them—old, young; thin, fat; uncanny, cheerful. The Greek conception tended to think the three Fates as being young, middle-aged, and old: the young one represents birth, the middle one life, and the old one death.
And then there's that Latin quotation at the end, which means "They who are about to die salute you." Traditionally, that's the greeting made to the emperor by condemned Roman criminals, gladiators, or anyone else who was, you know, about to die. (Sadly, it's probably not actually true.) Since the Fates control life, it makes sense for Conrad to throw this in.
But that leads us to the most important question: what happened to the third Fate? Which one's missing—life? birth? Is this an indication that something's gone horribly wrong?