my brother and sister-in-law, don't allow the kids to celebrate Halloween, taking them out of public school when the parties happen. I suspect this is because of crazy mother-in-law.
Are your brother and SIL deeply Christian? If not, why do they not have the spine to tell the MIL that she needs to keep her nose out of their business? I digress on your personal family issue ... I just hate parents without spines in the face of domineering grandparents ... it just gets worse over time.

Somebody should get the domain JesusWeenie.com and then hack JesusWeen to redirect to JesusWeenie showing a modern Christian version of JC with an erection. I have the mind of a hacker/anarchist, just not the resources.
When I was a kid, the school system allowed us to have Halloween parties, mostly in elementary school (early 1970s). By the time I made it to junior high (yes, folks, that's what they used to call "middle school"), there was less of Halloween occurring in the schools and then by the time I was in high school it was an individual thing -- you could get into costume if you wanted to but nothing was organized for it. Some people were sent home for costumes that were too revealing, and a few got into trouble for brining in certain accoutrements, such as a mace. (Yes, boys and girls, weapons are not allowed in schools.)
By the time I had graduated high school, I didn't know of anybody (or just don't recall) who objected to Halloween on religious grounds. In fact, I had never even heard of a religious basis for Halloween to begin with. It was just a day/night to get dressed up in costume, tell ghost stories, scare people, consume candy or adult beverages and ... that is that. While I was growing up, the worst thing to occur were all the terrorist stories on TV about razor blades in apples or candy that had been adulterated by ... adults ... to poison children -- nearly all of which were later debunked as urban legends.
When I and one of my college classmates moved to the town where we now live, I discovered through her, as an elementary school teacher, that they were not allowed to celebrate Halloween in the schools. They couldn't even mention "Halloween" at all. If the teachers were to have a party, it must be referred to as a "Fall Festival" and kids could only dress up as historical figures (giving some educational spin, I suppose), there could be pumpkins but not jack o' laterns, there could be candy but nothing with peanuts
[1], colors in displays could include all fall colors but not black ... I could go on. I asked her WHY? She then told me that there are people who have religious objections to Halloween and the school system acquiesced to their demands. She explained how the religious objectors used their theology to argue against and it and I was appalled -- not only for their success in foisting their religion on the school system but also for their abject stupidity in not knowing where Christmas came from. They did a hostile takeover of one religion's holiday but not another?
A couple of years later, the school system entirely abandoned the idea of "Fall Festivals" because the non-fucked-up parents cried foul, and so now the local school system has no official recognition or celebration of Halloween or the generic "fall festivals". My kid did learn about Oktoberfest, which is appropriate since it is not a religious holiday and doesn't include supernatural things (unless you find women with big breasts wearing dimdls and serving bier to be
divine). Plus, there is an enormous German ancestry where I live, so it adds to local color and tradition. It's okay by me that they don't do Halloween celebrations in the schools, but very uncool how that came to be.
Partial disclosure ... I am now on a "family life" committee with the local school system as parental input on how the school system shall select materials and present discussions on a host of "family life" issues such as sex education, sexual offenses/assaults, drug use/abuse, HIV and probably a few other things that I cannot recall. I was quite pleased to find that all of the people selected (we had to apply) are not religious nutjobs, which is something that one of the board representatives positioned as "you are enlightened people, which is why you were chosen, but do remember that everyone we serve is not so enlightened and we need your suggestions on how we should deal with them" insofar has how materials are chosen, presented, etc. I was rather impressed by the first meeting. Essentially, I think the school system relocated its spine.