Letter and Aunt Marge's Arrival

Everyone who has read or watched Harry Potter knows he lives with his relatives the Dursleys. Petunia (his mum's sister), Vernon (his uncle) and Dudley his cousin. But what if he had another cousin. Who was like his mother a muggle born witch. What happens when she comes face to face with a werewolf on a full moon? Does she return to her family after her first year? Does she go to the Quidditch world cup and Triwizard tournament? Is she invited to the yule ball? How does Umbridge treat her? Does she join the DA or the Order? What does Voldemort what with her?