Lol, this "reveal" is such a nothingburger by Florencia. Of course Gu Xin knows her sister is monitoring her. Meng Nian is obsessed and has "tormented" her.
Plus, Gu Xin won't admit to herself that she likes Meng Nian, so Florencia trying to say "I'm the better lover" is so worthless because Gu Xin doesn't see either of them as potential lovers (consciously, anyway).
Also in case anyone is curious, dextrocardia is the condition where the location of the heart is inversed. So Gu Xin, knowing this from being the author, aimed for the location of a normal heart in order to deliberately not kill her. Florencia knows this and finds it odd.