Goodnight Mommy

I enjoy good foreign films and when I read about "Goodnight Mommy," I patiently waited for it to be available to rent. So I rented it. And I hated it. Not because it was a horrible film but because I didn't understand what was going on b'twn the twins and their mom. Was it all in their heads? Was she not their mom? There seems to be a lot of 'net chatter regarding the ending and I'm with them on that. Anyone?