My sister in law threatened me the other day saying that I can get arrested because I told my brother who was yelling at me and insulting me.

I doubt there would be an arrest, but I cannot imagine it coming to this, can you?
The bickering and fighting in front of a person already deep in suffering from dementia is, to my mind, very cruel. I hope that you ALL will make every attempt to do better, and--when you cannot control yourselves--leave the premises.

You know what generally happens when the police respond to a family fight? If one person goes to jail, everyone goes to jail, because fights like that are rarely one-sided; and many agencies have a "zero tolerance" policy that specifically states everyone gets arrested, and then the courts can sort it out.

How on Earth will that help your sister?

If your relationship with your brother has devolved to the point where it is so acrimonious you are threatening to call the police on each other, don't you think a better solution would be to not spend time with each other? Only communicate via written documentation: ie. email? Isn't taking care of a family member with dementia enough drama to deal with, without inviting more?

Most likely, none of us should go where we’re not wanted. Police are trained to deescalate situations, many of them family dramas. They really don’t want involvement on either side. Accepting what you cannot change is a gift you give yourself