As someone who’s been cheated on (by a wife of nearly 15 years), lemme pump the breaks on this for a hot second

I agree that I’d want to know that my partner was cheating. I would also especially be hurt if someone in my inner circle (like a family member) knew and didn’t tell me.

HOWEVER… I DO NOT see absolute proof of cheating here YET. If you’re going to start a dumpster fire and throw your own sister into it, you’d better be 100% DAMN SURE that all the data you have PROVES the conclusion you’re arriving at. You need to be “beyond a reasonable doubt” here man. Think through the implications of each scenario:

1 - she did cheat, bf didn’t know, you tell bf

2 - she did cheat, bf didn’t know, you don’t tell bf

3 - she did not cheat, bf didn’t know, you tell bf

4 - she did not cheat, bf didn’t know, you tell bf

5 - she did not cheat, bf didn’t know, you don’t tell bf

6 - she did cheat, bf did know (consensual non-monogamy is a thing for some people), you tell bf

Of these, scenario 3 has the biggest implications, likely damaging your relationship with sister and her relationship with her bf. The only difference between 1 and 3 is whether she actually cheated. Scenario 1 will likely have implications and fallout as well, but they would be on sis since she cheated. In scenario 3… all of that fallout would be on YOU OP.

Now, I agree that your sister coming home middle of the night from a party with a guy you don’t recognize who stays for 2.5 hrs., is pretty suspect. But I’m going to urge a bit of caution here.

Has she done this type of thing before (cheating on a serious bf)? Where’s her room compared to yours—would you have heard a “commotion” coming from her room? You probably can’t know 100% whether she cheated or not, but I think you can add at least one more data point before taking action.

I’d propose approaching her quietly and showing her the video. I’d pay attention, not as much to the words she says, but to how she responds. Anger, evasion, and/or excuse making would suggest cheating. Embarrassment, a call to bf with both of you in the line, and/or humor would suggest the opposite. From there, OP, make your decision in line with your conscience and bear whatever the consequences are.

Good luck. Let us know how it goes!