Question: how are ser jorah and lyanna related?
Answer: As revealed (and made quite clear) in Game of Thrones season 8, episode 2, Jorah and Lyanna are cousins; Jeor and Maege Mormont were siblings. When Jeor voluntarily joined the Night's Watch, he relinquished all titles, including being the head of House Mormont. And so, Jorah became the new Lord of Bear Island.