Accidentally exposing my sister snap chat

Sorry for the long post!

my sister (18) has been living with me and my husband past 2 years as my family is messed up. She had two boyfriend that didn't work out and she promised to focus on school (1st year engineering) and not get a boyfriend. My husband and I put our lives on hold to care for her, get her into university, feed and provide shelter.

She's a good girl but a LIAR! LIES EVERYTIME. LIES FOR EVERYTHING.

We have been honest with her and told her no more lies. She has basically failed first semester however was "studying" 12 hours a day with full makeup, coming home 2 am etc.

She's been telling us that her best friend wants to bring her food etc however accidentally told she's dating him since April 2020. Ok that's cool but I know she was lying. So in front of my husband I told her to show me her snapchat eyes only as this is the secret photo teens have. I wanted to prove to her that she's lying and have been dating for a while. So she has intimate pictures there with her bf. Now I feel terrible I did not realize she was already having intimate relationships. Thing is this made her look really bad but I just wanted to prove that she is lying because clearly they have been dating since school started.

It was wrong for me to do that, I don't have kids and I have to be a parent to my sister. I am trying! I don't understand why she continuously lies when she could of told us and we are ok with it. If she is out of our home 100% she will drop out. My husband and I fight because of her lies..

Any advice??