So. Hear me out here. Get the sis prego then during late 3rd trimester have her transform into futa mode. What happens to the child? Where can it come out from? Does sister just rupture and bleed out? Does she shat it out? Or instant aborto upon transformation?
I don't know if I'd call her evil exactly. She is a low-talent burnout that has undoubtedly failed numerous times but is still desperately clinging on to her career no matter what she needs to do. Sleep with the boss, lying, emotional manipulation, guilt-tripping, trying to trick even youngsters...
Probably in something like this order: Beast girl would go nuts and murder cleric/saint. Party leader would go nuts and murder MC. Beast girl would go nuts and murder party leader. Loli would go nuts and murder beast girl.