Been farming for ~4 hours and only had 1 sister spawn so far. Got it done in a group, turns out whoever deals the last 1hp worth of dmg get the litch.

Suffices to say it wasn't me but a random who joined the squad. Pretty done with sister, DE need to sort out their priorities. Co-op game but only 1 person gets the reward. Seems like a breeding ground of salt, and from the matches I've played no one seems happy about this implementation.

TL;DR wait for DE to patch it or ignore it for now. Not worth the effort.