AITA for telling my sister her fiancé might not be as loyal as she thinks?

A 24-year-old woman witnessed her sister’s fiancé acting suspiciously while working at a bar and decided to share what she saw. Her sister didn’t take it well, accusing her of trying to ruin the relationship. Family members are divided—was she wrong to get involved, or was she right to tell the truth?

‘ AITA for telling my sister her fiancé might not be as loyal as she thinks?’

My sister (27F) has been engaged to her fiancé, Jake (30M), for about six months. They’ve been together for a few years, and while I’ve always supported her, I’ve noticed things about Jake that make me uncomfortable.
For context, I (24F) work at a popular bar in our town, and Jake comes in with his friends sometimes.

A couple of weeks ago, he showed up with a group and was very flirty with a woman who wasn’t my sister. He had his arm around her, bought her drinks, and at one point, I saw them leave together. I didn’t want to jump to conclusions, so I tried to let it go, but I couldn’t shake the feeling.

Last weekend, it happened again—Jake came in with the same woman, acting just as close. I felt sick watching it, and I decided I couldn’t keep quiet anymore. I told my sister about what I saw. I made it clear that I wasn’t accusing him of cheating outright, but I thought she deserved to know what I witnessed so she could decide what to do.

She got really upset and accused me of trying to ruin her relationship. She said Jake has been nothing but loving and loyal to her, and she doesn’t believe he would ever betray her. Now she’s barely speaking to me, and some family members are saying I should’ve kept quiet because it’s not my business. I feel like I did the right thing by telling her, but now I’m wondering if I overstepped. AITA for telling my sister her fiancé might not be as loyal as she thinks?

Let’s dive into the reactions from Reddit:

Couette-Couette −  NTA. You were right to tell her but let it go now. You warned her and she decided to not investigate it further. That’s her call.

thenicekittykitty −  You did exactly what Jake wanted you to do. He knew you worked there and would tell you sister beacause. He wants out, but is not man enough to do it himself.

Creepy-Stable-6192 −  NTA. Now he will know not to go to that bar. If you have friends who work at other bars, get them to take pictures. Or if your bar has cameras, ask to look at them and get the period of time he was there to show your sister. Rose tinted glasses are hard to take off.

VirieGinny88 −  NTA for telling her, she deserved to know. At this point though. you’ve tried and she wouldn’t hear you, so now let her make her own mistakes.

Electronic_Ladder398 −  NTA, but things like these are very hard to convince unless you have evident. Next time it happens again, maybe you can take a picture when he has his arm around her or something then show it to your sister. If she’s still in denial, at least you tried your best. BUT, doing this could cost you your job, so think carefully before doing it.

dijetlo007 −  Does he not know you’re his fiance sister?. This all seems a little odd.

jenasmiles −  NTA, you told her, she denied, let it drop and be ready to support her when she realizes what he’s doing later.

Bronze-Glass-2681 −  NTA. Your sister is lying to herself.

lonly25 −  Damn if you do damn if you done. I would have told. For the record I would have taken a picture. That’s just me.

Imaginary_Chair_6958 −  Either there are a lot of Jakes out there or this is another fake Jake story.