I'm involved in a lawsuit right now. As stated, you would have to prove your mother was not of sound mind and coerced into making the new will. There are laws against that and it is called elder abuse. If the congregation was involved in the coercion, that is an additional factor.
Get all the facts together the best of your ability, and make a list of questions. Ask around for a recommendation of a good estate attorney, be careful of those that work pro bono as they usually take a healthy percentage of an estate. Make sure the attorney you use is clear about the anticipated costs, court etc., and state their hourly fees.
Also, taking legal steps will force your JW siblings to also retain counsel which is costly. The goal is mediation which is less costly than court for both sides. It may cost $1-2K to file a protest to the will, but if your JW siblings see you are serious, and they may have to prove in a court of law why they had your mother change her will, they may seek to settle.
I didn't know any of this but asked around, got good advice, and luckily found an estate attorney. My case involves a black widow who really deserves a Dateline episode.
Good luck.