 If you liked
Pietà
|
 ...then you might like
Ane no Shinyuu, Watashi no Koibito.
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Both manga tackles the topic of depression within the context of a romance, both avoiding bad tropes like love triangles or needless drama, and going for a carefully constructed and nuanced study of emotions and the way they can evolve.
While Pietà is overall a bit darker in tone and has teenage protagonists, "My Sister's Best Friend, My Lover." has an adult cast.