First, sorry for the loss of your niece. It's never easy losing someone so young.

Second, I agree with the others about donating what you comfortably can. If donations have been set up to go through a bank or the funeral home, they simply notify the family that a donation has been made, most don't reveal the size of the donation. Even if that isn't how it's been set up, I don't think that anyone who has set up a memorial fund scrutinizes the amount of the donation, they're just happy/grateful that donations are being made for the cause.

Since you're a creative person perhaps you could find a creative way to add to the memorial's finances and publicize it at the same time. Maybe do a piece of your art that could be raffled off? Get the local media involved in the project to make people aware of the memorial fund and Huntington's.

Again, my condolences.