My Mum passed away 5 years ago..my daughter graduated last May from University and my Mum had longed to live to see that day. My daughter missed her terribly not being there..it was all perfect except for that.
She always gave her grandchildren money when they graduated University...on that day, I gave her a card with the money inside and wrote that 'Nanny' is watching over you today and her heart is full with pride and love. On her behalf, we carry on her wishes and her gift'
I gave it to her quietly..near the end of the day. It really was just a moment of reflection and rememberence for us all. And I know, Mum was there.
I knew my daughter would be thinking of her and I am sure your niece will be thinking of her Mom as well. I would take a quiet moment to ackowledge that she stands beside her, always.