Complete breakdown in relationship between mother and daugher

As Allsorts has posted, it's too late to be able to find out why this happened. For some who estrange family members, it doesn't feel appropriate to go to the funeral of the person they estranged and wouldn't be welcomed by other family members if they did.

Try not to dwell on this.

What did they say about each other?

I don't have any answers sorry, some people just don't get on but maybe there are clues in what they told you about each other?

It’s too late to find out, they both were obviously happy not to see each other. I find it strange you never tried to find out when they sounded off at you about each other, wouldn’t expect sister to attend the funeral if she hated her so much.. Neither come out of it well.

It is not surprising for a person who goes NC not to attend the funeral

I dont think anybody here can tell you much how it came about

It can be many things: incompatibility of personalities, cannot agree to disagree on something, etc.

This sounds a complex situation and I suspect there is more to the story than you know. Are you able to speak to your sister and find out what on earth has happened?