Do you and your sister enjoy each others company? How about suggesting, as she has most things already, you treat the both of you to an enjoyable day out together.
Give her/ask for some ideas. Some suggestions (that I would like).
Looking around an exhibition/museum/gallery then lunch. (think some may be opening soon?)
A tour round a historic house and gardens near you, and afternoon tea of course. www.historichouses.org/tour-listing.html
A workshop to learn a craft, felt making animals, mosaics, glass etc.
Cinema trip (now some open) with meal
Boat trip, could be watching seals, wildlife etc.,
She is your sister and this is a special Birthday, so try one more time, to make it a lovely day for her.