Oh no...Harry and Hermione do not have a mother/son relationship. 😛 That's saved for Mrs. Weasley/Harry. Hermione's role plays into that slightly, but her relationship is not mother/son.
There's a reason people are jealous of the relationship Harry and Hermione have, and it's not because they act like mother and son, or even brother and sister. It's more complex than that. They aren't lovers or romantic, obviously (we've known that since book 4), but there is more touching between them than you see in a typical brother/sister relationship.
There's no real word for it that I can think of at the moment, but I would never put it as mother/son.
But back to the original topic (if we want to discuss Harry and Hermione's non-romantic relationship, should we start a new thread?), DH is going to mutilated, but hopefully not beyond recognition.
I see them squishing quite a few scenes together in order for the whole story to get across, but even if they cut out a whole lot of the book, I still see this being a 3 hour movie.
Scenes I think will get cut are things with Tonks and Lupin, most of Voldemort's search for that one particular item he's so keen to find, and I think Kreacher's scenes will be changed drastically in the film.