Should i let i be ****** up sister?

my friends told me a couple months ago that my older sister was dating my ex boyfriend i thought it was funny cause hes only 16 and shes about to be 20 i really didnt care about it til 3 months p and one of my other older sisters told me that my 20 year old sister was hang out with one of my ex so days p and i caught my ex walkin my sister home when my sister seen me she didnt know what to say so asked her if she was dating him that it doesnt matter as long as she tells me the truth she said no so she denied it til the fullest 1 month pes by i go to her room and i find a picture of him and her kissing i started laughing cuz i really didnt care but she was mad at me for a while like if i was the one who did something wrong?should i forget about it and let it be or do something about it