The Sewn Sisters are a coven of Night Hags who minded the Soulmonger.
Commanded to do so by Diath, Shemeshka gave the Wafflecrew a way to summon them so that the sisters could help protect the heroes from the death curse.  Once the group got to Port Nyanzaru, they summoned the sisters, who visited them in their dreams on three consecutive nights. On the first night, the hags "rode" Diath, Paultin, and Strix in an erotic manner and spit into their mouths.  On the second night, they stuffed sacks into their mouths.  On the third night, the hags cut open the Wafflecrew's torsos, removed a larva from the sacks, then put the larvae back in and sewed up the wound. 
The hags created evil clones of the heroes, using hair from the real Wafflecrew that Strix had baked into a pie as a peace offering. The hags sent the clone party to Port Nyanzaru to magically transport evil beings back to them through a soul bag. The Wafflecrew dealt with the clone party - as well as an evil "clone" of Construct Evelyn of unknown origins - killing all but Clone Paultin, who escaped.
The real Wafflecrew met with the Sewn Sisters and gave them some more of their hair in the hope that the hags could bring Evelyn back to life. The hair was exchanged for the Sewn Sisters obtaining Mr. Fox's spectacles, X the Mystic's Arcane Grimoire, and a resurrection scroll. 
The sisters tried to get Strix to agree to carry a child for them in exchange for retrieving the missing piece of Diath's soul. Strix was eager to offer herself up for Diath's protection, hoping that claiming back the piece of Diath's soul would place him out of Shemeshka's reach (despite Strix not knowing how babies are made or knowing entirely what she would be getting into) but Diath refused on her behalf. 
The Sewn Sisters
Peggy Deadbells: She has a peg leg and spews noxious gas when she laughs.
Widow Groat: Coins that cover her eyes, and ants crawl out of a hole in her head.
Baggy Nanna: She has a sack over her head in which animals dwell.