Megan Jenkins is a young woman living in Wales, evidently in our world or one very much like it. Her brother, Howell Jenkins, has just walked out on the family at some point, and reappears at intervals. Megan doesn't realize it, but it is because he has received extensive training as a wizard in a parallel magical universe, and has become the Magician Howl there, living in a wandering magical castle that ambles from place to place, making people nervous. It is anchored to several places, and the door has a mechanism that can be set to take it to one of these places at a time. The world and Welsh town where Megan lives is one of the anchor places.
When we see Megan, she is basically nagging and complaining all the time. It's quite clear why Howell left the place as quickly as he was able to.
Megan has two children: Neil and Mari. Howell is rather fonder of them than not, and is never unhappy to see them. However, they succeed in complicating the plot of Howl's Moving Castle because of Neil's teacher Lily Angorian, who is wicked and powerful and interested in Howl and his world.