Buzzrelly, I'm so sorry to hear of your loss. Your niece was lucky to have a family who cared so much for her, as well as your mom. Prayers go out to your family over your loss.
We are going through a situation which although a bit different, somewhat similar. My brother was diagnosed with stage 4 colon cancer last week, my mom and dad in their late 60's(they are divorced) are his main caretakers.
We have had a trip planned to Disney with both my mom and dad for a while, and then this happened. The trip won't be cancelled-it is my daughter's senior trip and she's been planning it quite a long time, but my brother will be going through chemo and recovering from a surgery he had last week. My mom, usually the one who insists on taking care of everything and everyone, decided that when my dad offered to cancel and stay home with him so she could go, she would take him up on it. In her words, she needs the rest and the time away before a possibly long fight and alot of taking care of my brother-this is totally unlike her. If she says she needs a break, she means it.
I hope your mom takes the time to go with you. Maybe you all could do something while there in memory of your niece...let a balloon go, do something she loved, etc. I hope either way you have a wonderful trip as your niece would probably want you to.