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.
The Woodland Trust will plant trees in memory of someone. You can choose one of their woods significant to you and you can then visit it and see the beauty of your memorial and that it is contributing to a better, greener world.
Sorry to hear your unhappy situation but good that you can talk about a special way of remembering your sister in the future. I always think trees are a great memorial but it might not be possible to plant in a place where all who love her can visit. A friend who was keen on running arranged for a trophy to be given to the running club to be awarded annually..I cannot recall the category..but her name is said every year in a place that meant a lot to her.
I am about loose my creative, energetic and talented sister. We are using part of this time to discuss what creative memorial we would like create for her. While a seat in Harlow Gardens is the obvious choice, I am concerned it will only have a plaque - and I'd like something more fulsome. While walking at Loch Lomond we found beautiful wreath of fake roses placed on a tree in 2019 adorned with small hearts inscribed with 10 messages from immediate family members! This struck me as a lovely idea. Does anyone have any other creative ideas?