Tell me the film I see I see?

Hi, l recently watched the movie "i see i see" on Netflix. Unfortunately I did not quite understand the movie. It's about the kids thinking that's not the real mother, and I'm not sure if she's that or not. Or one brother (Lukas) died supposedly but how can he drink a glass empty? Luke is really just a fancy of elias. Can someone help me please?

https://de.wikipedia.org/...h#Handlung