I am SO very sorry you are dealing with this. It's hard enough managing the care of parents in their decline without hostility and drama from a sibling. As for the question of "when" — it depends so much on the particular situation. But the biggest mistake people make is to wait too long — and then they're responding to an emergency, not a plan, and their options are fewer. IDK if you'll be able to disrupt the dynamic between your partner and the bullying sister, but you can and must draw your own boundaries. I know, it's way easier said than done, but your health is at stake here, on multiple levels. You may be able to find an elder care ombudsman or consultant who can help with this and give you and your beloved some distance.