This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Sister Princess
Episode 25. Despite all the good times with his sisters, Wataru still decides to leave Promised Island with Akio whom he looks up to. This moment includes the reactions of all the sisters and Wataru seemingly said he's not going back.
Another moment in the series is in Episode 3 where Karen talks to Wataru about her thoughts of having siblings. The way she walks away after that with hands over her eyes is so sad, as is Wataru deciding to stay in Promised Island and walking back home together with her.