I'm having trouble rectifying something,
my Mum's requests upon her death were few. The two things most important to her and which she was emphatic about were (1) there be no viewing, she wished to be remembered as she was when she was living (2) she be cremated "mesquite over the coals".
During the many meetings with the funeral directors we explicitly told them her wishes. They told us that the day before the memorial service my brothers, my sister and me had to come to the funeral home and see her for legal identification purposes. My sisters daughter knowing what time we were to meet at the funeral home took it upon herself to go up to the funeral home, with her boyfriend, before we arrived and view my Mum.
During the days preceding her death, my Mum's wishes came up many times in discussions. My niece had been party to many of these conversations and she stated she didn't agree with what her grandmother wanted and we told her what she thought or wanted was irrelevant, what her grandmother wanted was all that mattered.
I am angry that the funeral personnel let her view my Mum. I am angry at her for circumventing my Mum's wishes. This situation really sticks in my craw. I probably sound like a real sod but a midst everything else that brings me to tears in a flash, this thing is right up there. I feel like I let her down 
= new reply since forum marked as read
...run-ins within my family. Nosey, take charge when they should shut up, pushy and unpleasant.
I finally told one of them. "I'm the Son..do you understand?.."If you ever pull a stunt like that again, I'm going
to cause you to never, ever want to do it again"
I was pretty damn nasty about the whole thing...but they have backed WAY off. 
You did your best. I understand that you're upset at not having been able to comply with your Mum's requests fully, but it's not on you at all. And you do NOT sound like a sod; you sound like a real person.
please hang around with us; you surely don't deserve to feel like that.
I miss my Dad all the time; he passed on last April, and I'm planning to visit his resting place soon. I think of what we shared, what he taught me, and hope not to be caught feeling like you do.
Let it go. Your mother was not hurt by this. Your niece is living. Your mother is not. You kept faith with your mother's wishes, your niece said a private goodby, and it is time to move on; stop fretting about this - it sounds like you are focusing your emotional energy of Grief and being sad about a loss that you couldn't control into being angry at someone which you can control and create drama over.
Sometimes anger is less painful than grief, and makes a nice distraction from pain, so we create drama where none needs to exist. This sounds like one of those situations.
Mourn your loss. Unless you wish to bury your relationship with your niece with your mother's remains, let it go. Trust me when I say that a "discrete private farewell" doesn't even make the top one hundred list of shitty things people sometimes do when it is time to say goodbye to a loved one.
Let it go. 
ON EDIT: condolences on your loss.
A sod you are not. Not everyone can experience the emotional ties with their parents that you do. So heres to your humanity.
The pain and turmoil you're feeling now is natural. You didn't let your Mum down.
The crassness, rudeness, and ignorance of your neice may not go away. But for the sake of your sanity, please learn to let it go. There probably isn't any amount of rectification, explanation, or thoughtfullness that you can share with your neice that will change her mind or attitude. One may hope she grows out of it. One never knows...
I'm sorry for your loss. Please be sure, during this time of grieving, to remember the entirety of her life and yours together.
If you need someone to talk to, we're here.