Because Sigyn wasn't in the movie, and her only appearances in the comics show her in strange and unwieldy (and non-matching) outfits, Darling designed her Sigyn costume herself based on Alebireo's fanart.
The costumes itself was altered from a large black maternity dress. It was retailored into a V-cut, low backed dress. Criss cross wraps of satin were sewn on as well as chiffon sleeve drapes and trails. Sigyn's headress is made from sintra and the rest is completed with jewlery and a good wig.