We know Murasaki Shikibu had an older sister from records of the children born to Tametoki and his first wife, and from this heading from one of Murasaki's poems:
“My elder sister died. I met someone whose younger sister had died and she suggested that we should think of each other as the lost relation. I wrote to her as such, and she to me. Both of us left for far-off lands; I wrote in longing:
Kita e yuku/ Kari no tsubasa ni/ Kotozute yo/ Kumo no uwagaki
Send word/ By the wild geese/ Flying north;/ As often as they wing the clouds/ Write constantly.”
(Richard Bowring translation)
Other than the fact that she existed and died relatively young, nothing is known of this older sister, not even her name. The idea that she was mentally retarded is my own invention. I named her Takako by coincidence, before I learned that some scholars think that might have been Murasaki's given name.
The poem above, with the sliver of information in its header, provided the inspiration for Takako's death scene and the creation of the character Kerria Rose as the "someone" Murasaki met at that time.