I almost feel bad for upvoting this post because it kind of feels like I like it.. but first of all, my condolences for you and your family.
It's great that you are writing something, and I'm sure it will be appreciated by those present at the funeral. Are you going to read it yourself?
I was 16 when my grandmother died (she was like a mother to me) and like my mother and aunt, I also wrote a poem for her funeral. I couldn't read it there though, so the priest did. Looking back, I'm glad I said I wouldn't do it myself as there were hundreds of people there, apparently many people I never met were there so I would have chocked in my own words probably.
It's always hard to find the right words, just wishing you and the family strength and love.. take care!