Both are freedom fighters, and both seek to remove an Imperial ship from the Ryloth system.
Bits of dialogue reveal that the two were once as close as sisters and that Sabine was also a bounty hunter.
But she can level with Sabine about the gravity of the situation, show understanding to her feelings, and offer guidance.
This is where we get the backstory of Tarre Vizsla, the first and, so far, only Mandalorian Jedi, crafting the blade, and how it was stolen back from the Jedi order by the Mandalorians.
On the other hand, Rebels exists to introduce and develop its own original characters and show the transition from small, ragtag rebel cells into a larger, connected rebellion.
The mysterious nature of Fulcrum represents the secretiveness of the Rebellion as a whole, something that troubles the young woman who wants more trust and honesty.