Another moment when Satsuki’s sister senses were tingling. Even though she didn’t know they were related yet, she still shows up and stops Nui from killing Ryuko. At this point, Satsuki probably didn’t even value or need Ryuko anymore. Senketsu was chopped up and she would be useless without him (at least to Satsuki), so keeping Ryuko alive would be pointless. Instead, she shows up ready to just straight up kill Nui.
Satsuki: There’s only one way to deal with those who choose to disobey my orders.
Nui: Oh my gosh. Now what could that be?
Satsuki: *unsheathes bakuzan* The blood purge. Prepare yourself!!
At that point, Satsuki was just tired of Nui always screwing up her master plan with her psychopathic whims.
Satsuki still scolds Ryuko for making her have to go all the way over to the slums to save her neck by calling her, “Pathetic”. Then she snatches all the leftover pieces of Senketsu.
She could have just left her to Nui’s nonexistent mercy, but no. She, unknowingly, saves her lil’ sis in her time of need.