It's already clear that Bo is not Sabine's mother. But because I want them to be related somehow, I have a headcanon.
Sabine is not related to the Kryze's at all, but Sabine's mother was a Death Watch. As Rau said, Sabine could've been a protector—or a Nite Owl.
Sabine's mother sends Sabine to train as a Death Watch soldier, but Bo notices that Sabine is very talented, and takes her in as her Mandalorian apprentice.
When Sabine left Mandalore to join the empire, Bi was very disappointed. She saw in Sabine what Pre saw in Bo, she could've become the Leader of the Nite Owls.