So the literal mess is: A plausible interpretation: “My first love is the act of seducing an older sister — and it works.” Or more darkly: “The ‘big sister trap’ is my first love’s method of operation.”
"Anahemu Ore no Hatsukoi" is a great read for fans of: anehame ore no hatsukoi work
She looked at the postcard as if trying to coax a shape from fog. “I don’t know,” she said. “But waiting without living is a slow ache. You must follow what you can.” So the literal mess is: A plausible interpretation: