Many issues at hand, people who say forget about it bla bla to me that's gobbledygook. I remember when I was cheated on and I only found out when I was pissing razor blades, just imagine if it was hiv, or herpes or something nstead of a easily treatable stds. Now just imagine if this dude is sleeping raw with women of tinder a known hookup app, just imagine if he contracts hiv and hands it to your sister, could you live with that?
I'd tell your sister, however if you want to protect yourself then screen shot and send an anonymous e-mail or mms.
again the exterior and what you see is many times a facade, there is this guy at my church who is beating his girlfriend and mother of his child all the time, yet i'll see him on sunday all smiling, hugging his girlfriend. Hell I used to smile, be lovey dovey and post pictures on social media when I had it of me and my ex, even though on the inside i was stressed the hell out, many people wear masks.
Thirdly she met him off tinder eh? this is the problem in this generation when you meet people off these apps and dating sites, as soon as they get bored or a relationship gets a little mundane or stale ala novelty wears off honeymoon period of 3 months is gone, they go back to their familiar grounds ala dating sites and tinder looking for their next hit.
Fourthly even if you tell your sister and she confronts him, because of the emotons your sister prob has invested in him, she will accept the lie of, "I just forgot to delete my profile, my friend was just using my account, here take a look" jibberish and lies.
if they are on and off maybe he just weighing his options
(Hell and when I get a day off I'll be asleep at 3:30 in the afternoon)
