Not much is known about Sigyn.
In my eyes, Sigyn is one of the strongest in the Norse Pantheon. Not because of physical strength or powerful weapons, but because of her mental strength, stability and patience.
Sigyn is known as The Child Bride and The Mourning Mother.
The Child Bride describes her as such because it was told that she wed Loki when she was a child. The two loved each other of course and they love each other today.
They call her the Mourning Mother for a totally different reason. In the lore, Loki killed Baldr. The god of light. After his murderous deed, Loki fled Asgard. The rest of the gods soon found him and gave him a number of punishments.
- Odin and the other gods forced Loki and Sigyn to watch their two children, Narfi and Vali, meet their ends. Vali was transformed into a wolf and in turn devoured Narfi.
- Loki was bounded to a rock with Narfi's intestines.
- The Norse Goddess Skadi planted a poisonous serpent above Loki to drop stinging venom.
Sigyn now sits above Loki with a bowl, catching the poison. Everytime the bowl gets full, she turns to the side and dumps it. When the poison does hit Loki, it is said that the earth shakes rapidly.
If you are further interested in reading about Sigyn, here are some links provided.