Complex sentence: Eun-suk is devastated not only by this news because societal norm doesn't allow for romantic relationships with the deceased or divorced persons siblings, but also when she sees Min-gu in his current state.
Split into simpler sentences: Eun-suk is devastated not only by this news because societal norm doesn't allow for romantic relationships with the deceased or divorced persons siblings. She is further devastated when she sees Min-gu in his current state.