Sienna was the former High Leader of the White Fang.
Back when the White Fang was led by Ghira Belladonna, Sienna and Adam understood one another, shown when she nodded for him to engage the humans attacking their convoy. She openly supported Adam's methods and gave him control of the Vale branch of the White Fang.
In "Dread in the Air", their relationship was shown to be strained as Sienna criticized his involvement in the Fall of Beacon, citing how it actually hurt the White Fang in the long run. Once she realized Adam was trying to overthrow her, she defiantly proclaimed she would never serve him, right before Adam took her life and her position as High Leader.
Not much interaction has been shown between them, but during the convoy attack, it is shown that their different opinions on conflict resolution hurt their working relationship, resulting in Ghira stepping down as High Leader and Sienna taking his place.