Final answer:
Josephine is worried because she detects her sister's strong emotional response, assuming it to be grief, whereas the reader knows Louise is feeling free and euphoric. Louise's death, attributed to joy, unknowingly springs from a more nuanced internal struggle that culminated in a fatal shock when her husband returns home unexpectedly.
Step-by-step explanation:
The character of Josephine is worried because she senses her sister Louise's profound reaction to the news of Brently's death, but she is unaware of the actual cause of Louise's emotional state. The reader, in contrast to Josephine, is privy to Louise's innermost thoughts and the sense of liberation she feels when she believes herself to be free from her marriage. We understand that Josephine misinterprets Louise's reaction as grief, without grasping the complex feelings of relief and freedom that her sister is experiencing amidst the sorrow.
While Josephine believes she is protecting Louise from herself by imploring her to open the door, Louise is actually reveling in the thought of a newly independent life. This dramatic irony creates a tension in the narrative, heightened when Brently Mallard returns home, unaware of the reported accident, leading to a shocking twist where Louise's newfound joy is suddenly cut short, and her death is mistakenly attributed to 'the joy that kills' by the attending doctors.