Brother lives in house with aging parents. Last week my father died at home, family was not notified of death until cremation was done. Why?

I am sorry for your loss. See how brother is doing at this point and plan a future memorial.

Apparently this was sudden, no illness with family members in touch daily?
Best way to find out why he would do this would be to ask him.

Well, one of two things: pitch a fit and ask why you weren't notified (were you close? Did you visit often? Were you a 'helping' part of the needs of elderly parents?)

or

Don't pitch a fit and simply ask why you weren't notified. Simple question.

My mother lives with my brother's family. He is supposed to email us all every week simply a brief update on her health. In 22 years he has never done this once, despite us reminding, asking, begging for this simple 5 minute task.

I could see him doing something like this. He and mother have a weird and twisted relationship and although she complains endlessly about him, she also won't follow through on a single 'complaint' she has. We all just ignore it as trying to 'fix' this is pointless.

There must be an underlying reason. Just ask.

When my MIL dies I do no think that there are any plans to notify her oldest son, from whom she is estranged.

It's sad, but it happens. A lot.

I am so sorry.  Possibly brother was overwhelmed.  Had any siblings indicated to brother they would assist with final arrangements?