First of all, my condolences.
Anything I thought about the conflict you described has already been said.
I thought about quoting the ones I liked best, but then realized I'd have to quote *all* previous posts
I'd think you have two options:
a) let them sort it out;
b) play Ambassador, try and mediate a compromise. The "two separate services" concept *should* please everyone. If anyone will not take it, then they do not deserve to be there, period
From personal experience, JWs perceive their members as actual family, more so than actual blood family, hence the "ferocity" shown trying to have the service their way. They consider their faith far more relevant than actual family ties. They do not allow life saving blood transfusions, so I imagine they scorn DNA ties.
And the atheist sister, I'm willing to bet the reason she's an atheist and fended the JWs off, is precisely because of knowing them so well. If I hadn't had a JW grandfather, I would probably be a Catholic today.
I know this first hand because of my early life.
Since and because my father died in 1968, I lived with my mother and maternal grandparents in their house.
My grandmother was a true Catholic, but, my grandfather, during my mother's teenage years, converted to JW.
Because my GM refused to join him in his new found faith, they immediately moved to separate bedrooms, such was my GF religious inflexibility, so typical of JW.