I think it is unlikely that anyone other than the author could answer precisely ‘how many’ details of the story are autobiographical. But the author has revealed in interviews that there are specific stories from her family woven in, for example in this interview on Paulette Kennedy’s Blog Litteken says:
One of my favorite themes, in both literature and life, is exploring how the past affects our future, especially within families. The importance of preserving family stories, the ongoing emotional tolls of past experiences and traumas, and the connections that tie people together through generations has always fascinated me. There’s so much to be gained, even by seemingly random occurrences.
For example, my mom shared a story with me about how my great-grandmother used to give money to any police fundraising telemarketer that called her house. When my mom told her she didn’t have to do that, my great-grandmother didn’t believe her. She was truly worried that if she didn’t, her name would get put on a list or they would come arrest her in the night. Her experiences living under the oppressive Soviet regime impacted her so deeply that her fear still lingered, decades later. I recreated that scene in The Memory Keeper of Kyiv because there’s so much insight and empathy to be gained by recognizing what went on before, both good and bad. I think historical fiction is an excellent vehicle to raise that awareness.
Litteken also notes this incident in her Author’s note at the end of The memory Keeper of Kyiv, where she says that the character Cassie’s Bobby is in many ways her own Bobby, that she had married her widowed brother in law, but that the village she places Cassie’s family history in is fictional. But it is also clear from her Acknowledgements section that Lettekin isn’t a widow herself as her character ‘Cassie’ is.
It seems to be the case that while there are elements of her family’s story woven into the book, they are biographical details of her grand parents and great-grand parents rather than autobiographical details of her own.