As a POA, is it absolutely required that I give deadbeat siblings updates on my mother in-law?

I agree with AngieJoy. A brief email once a week would be a good idea.

Even if they never acknowledge your communications, I would go ahead and notify all of them via e-mail whenever you take your mother-in-law to a medical appointment (unless it's something very routine, like a monthly visit to the doctor's office for a B12 shot). The e-mail could be very brief. Here's an example: "Hi, all, Just wanted to let you know that we took Mom to the doctor for her quarterly checkup today and the doctor says that she is doing well"; "Hi, everyone, Just a quick note to let you know that we took Mom to the ER today because she was feeling very ill. The problem turned out to be a UTI. She was not admitted to the hospital. She's home now and taking an antibiotic." I've sent many such e-mails to my siblings. I can't offer any advice re providing financial updates. I have no involvement in my dad's everyday finances, even though I think that I should not be excluded. Don't want to hijack your question, so I won't go into further detail in this reply.