I get that most people make comments based on their own experiences, and from your I can clearly see you have never been in a situation like this. So I will share mine with you for the sake of understanding.
While we were both in college at the same University my sister was dating a guy none of us liked (myself, family, and friends). No biggie though, she can make her own decisions. Well he got caught on a security camera trying break into and robbing one of my close friends house's of abot $2k in guns and other items (this was after things had gone missing from my house and my parents, but we could not figure out who it was). Video was brought to me (after it was shown to the police by friend), I showed it to my sister (an all around good person, never any crazy stuff, never much of a date a bad-boy type).
My sister said she was breaking it off with him immediately, as a bunch of things fell into place when we all watched the tape and remembered very similar things about how our stuff went missing and how he had interacted with my close friend prior to the break in.
Fast forward 3 months; My sisters bank account was tied to my parents account for ease of transfer and overdraft protection. She was overdrafting every week by hundreds of dollars. My parents call her and she said she isnt spending anything. She just assumed she had money becuase of the over draft protection and never checked her statements, meanwhile it was pulling dollar fter dollar from her account while she thought she was the only one using it. Parents call me worried its drugs or something crazy, I put them at ease and said I was pretty sure it wasnt that as we would go out together and I knew she was adamantly against that life style.
Then they look at the transactions and see that its ATM withdrawals so call the bank and ask the bank to pull security footage.
Crazy crazy coincidence. Boyfriend was on the ATM camera making withdrawals every week from her account.
Hmmm, that's weird, how did he get her card? Turns out she was still seeing him, he would sneak her card out of her purse without her knowing and say he was going to pick them up food, would go to the bank, make withdrawals, then go get dinner for them....
It took his arrest for both that and the previous break in for her to finally realize that he was bad news. After the bank thing, we called in some favors and did some in-depth checking on the guy. He claimed he was in school at the University. NOPE. Had been lying about that, and when he was going to class, he was really going to campus and would steal and pawn bikes etc.
Sister was and is a completely normal person who just got wrapped up with a pyscho. And when presented with how bad of a person he was, she tried to "drop him" but just couldnt cut ties all the way and allowed him to stay in her life but in a very low-key way. He's served 3-5 years in state pen, and has a no contact order against him for our whole family. If he does, he violates his probation (of either 5-7 years) and goes right back to jail. Turns out when he was finally arrested, the police found evidence and property linking him to a ton of other crimes. Guy was a great manipulator and con man.
I'm not an over bearing brother, but I learned a valuable lesson. Sometimes you have to look out for family for their own best interest. They may not like it at the time, but caring for someone can make you blind to all their faults.
I never got angry with my sister, I just got worried. But i did not want to be that overbearing brother who looked over her shoulder and questioned her about the obviously bad guy all the time, I did ask about him and check in to make sure she was fine but I should have done more. I should have been that brother, it would have saved thousands of dollars which we could not afford to loose and never got back (and we are not rich by any means, both of my sister was only able to attend to school because I received a full athletic scholarship and then she received a financial scholarship, my parents had saved some money but not enough to afford to send either one of us to the state school) and a hell of a lot of stress and heartache on my family's part.
Hence, my off the cuff comments in a reddit post, from one brother to another, to check in with his sister.