I'm so sorry to hear about your family's loss.
A memory book that's designed for the family to fill in would be a wonderful gift.
Also, I know of a memorial web site where people support each other through the period of grieving. It doesn't have a tremendous amount of activity, but that can be good, because the people there really get to know and trust each other. You could go there and purchase a residency (a memorial page) where your cousin's wife can post her husband's picture, tell his story, leave flowers, etc. She and her daughter can leave messages, and other people can sign the guest book whenever they want to. It's only $25 a year for a residency, and they can really provide a healing place of safety and comfort for those of us left behind. The site is:
She can also copy and paste the messages into a computer file so that if she ever decides to let the residency lapse, she will still have all of those messages to put into a hard copy of a memorial book.
So, whether or not she and her daughter ever decide to participate in the forum at the site, the residency will be a place where they can have some quiet time alone with your cousin.
I also love the idea of planting a tree, shrub, or rose in memory of your cousin. I planted a rose (the name of which was especially pertinent to my dad) in my dad's memory, and it actually bloomed for the first tie on his birthday ~ January 19th! ~ in the dead of winter, the day after a snowstorm. If that wasn't a sign from above, I don't know what was!
My thoughts and prayers are with your family now.