Lady Catherine Grey [SEP] sister
Lady Katherine Grey
Katherine Seymour, Countess of Hertford (25 August 1540 – 26 January 1568), born Lady Katherine Grey, was a younger sister of Lady Jane Grey. 
A granddaughter of Henry VIII's sister Mary, she emerged as a prospective successor to her cousin, Elizabeth I of England, before incurring Queen Elizabeth's wrath by secretly marrying Edward Seymour, 1st Earl of Hertford. Arrested after the Queen was informed of their clandestine marriage, Katherine ("as" Lady Hertford) lived in captivity until her
death.
In Edward's Device,
1. Lady Jane Dudley (b. 1536/7), Frances Brandon's eldest daughter
2. Lady Catherine Grey (b. 1540), Frances Brandon's second daughter
3. Lady Mary Grey (b. 1545), Frances Brandon's third daughter
4. Lady Margaret Clifford (b. 1540), daughter of Frances Brandon's sister, Eleanor Clifford, Countess of Cumberland
Upon his death, the first person in line per Edward's Device was proclaimed Queen Jane. Within