Henry VIII of England had three siblings. His older brother Arthur died young, making Henry the heir to the British throne. His two sisters Mary and Margaret Tudor were married to the kings of France and Scotland, respectively.
Henry VIII married Arthur's fiancée Catherine and fathered the future Queen Mary. Mary Tudor's marriage produced Mary, Queen of Scots. Margaret was the grandmother of Lady Jane Grey, another claimant to the British throne. When Henry VIII died, he was first succeeded by his only son, Edward and, when Edward died young, by Catherine's daughter Mary.
Of his children, Henry's daughter Elizabeth had the weakest claim to the throne, as she was considered illegitimate by the Catholic church. For this reason, her cousins Mary, Queen of Scots and Jane Grey seriously endangered her throne. In the end, she displayed her father's ruthlessness and executed both of them.