I think adding to flowers or trees that already there a lovely idea. I personally wouldn't want something hung in a tree [it would detract from the beauty of the trees] and be personal only to the family, what happens when the family are no more. Or if thousands decide to do the same.
I'm not keen on naming a seat either and always find it unsettling when you get to a beauty spot and the seat is dedicated to someone, I reflect more on them than on the view I am meant to be looking at.
Maybe contributing to an area that is being rewilded in an area your sister loves. Either trees or plants or even animals like red squirrels or otters.