All of Grace's sisters have distinct motive for murdering JP. At one point in their lives, each of them have had personal conflicts with him that would warrant revenge.

As for JP himself, he is a cruel and abusive husband who mistreats Grace and their daughter and actively hates the rest of her siblings. Despite this, Grace has become distant from her family thanks to her need to defend his actions at every turn. The Garvey sisters take it upon themselves to save her from him.

Motivated by revenge and a notion of family support, the sisters begin plotting to murder JP, spending well over six months trying to concoct a plan.

In present day, we know that the deed is already done. Insurance agent Thomas Claffin (Brian Gleeson) is doggedly pursuing the case as a murder in order to salvage his failing agency. He quickly comes to suspect the Garvey sisters and tries to narrow it down to one prime suspect.

Episode 10, "Saving Grace," finally answers the question.

Despite the Garvey sisters' best efforts, they ultimately fail to actually kill JP. In actuality, it was Grace herself who was able to pull it off.

She reveals that one night (her birthday), she and JP try to have a romantic evening in. After the night goes wrong and JP cruelly admonishes her, Grace finally snaps under all of the pressure and strangles him herself.

Fortunately, all's well that ends well in this scandalous murder. Tom's brother Matt destroys any evidence leading to Grace in exchange for Grace withdrawing the insurance claim to save Tom's business. The sisters are then reunited in body and spirit, now free of JP's toxic behavior.

You can stream the whole first season of Bad Sisters on Apple TV Plus.