I think the 2 key things to know are:
1) Its autobiographical. Miyazaki sees himself as the old uncle guy, and the film is about how he is self-critical of his legacy, but takes solace if his work has been able to help comfort bereaved and troubled kids (the kid in the film who discovered his magical world is supposed to represent the collective Ghibli audiences).
2) The step-mum and aunt are the same person, and confusingly the kid calls that character 'mum', even when his real mother is there too. So basically the father shacked up with his ex-wife's sister. That's not a spoiler, its just a bit confusing
Its a more complex and challenging film than some of his, but i still though it was excellent