Getting to the heart of the matter, this sounds like your core pain:
it's not so easy to put all the feelings aside and ignore this huge hole inside where you want to have a loving family that celebrates a wonderful event like a wedding. I want to be close to my family and share things
It is sad, and also that you're being reminded of how much poor character they demonstrated at your wedding. How close are you to your niece? Can you talk to her? No agenda, just as a supportive Aunt? "Hi honey, I just wanted to reach out and see if you need anything." Or similar. Maybe she'll open up. Possibly she might connect whatever she saw at your wedding with the present.