Franz Kafka had many cousins because his father had 5 brothers and sisters and his mom had 4 siblings.
In his book, Kafka's Relatives, by Anthony Northey, there's no cousin named Fransiska.
But there's a Fransiska Kafka (nee Platowski), born 1816, died 1880, which actually the grandmother of Franz. She married Jakob Kafka. I suggest you post this question also on the JewishGen website. Do you know how?