My sister is in a relationship with my husband’s brother

Kindly hide my identity and inform me when you post this. I am an ardent follower of your page and I like how your readers contribute. I have an issue which I would like to talk to you about. I am a married woman. I have been married for three years and I have a daughter. My husband is a good person and I am enjoying my marriage so far. I am having a challenge and it has to do with my sister and I need your advice. My sister is 23 and she is a university student. She is the last child of the house and I have four other siblings. Everyone in my family loves her and treats her like the last daughter she is and when it comes to money, she doesn’t lack because many of us are doing well so we send her enough money to take care of her needs. My husband on the other hand has a brother. He is no longer a student. He works with a company here in Abuja and to an extent, he is comfortable and earns well. 

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My sister came to visit me last week and during the short time she spent with me, I realized that she was keeping in touch with my brother in-law. It is not as if it is wrong or something like that but I found out that it was more than just a casual keeping in touch. Often, when I talk with her, she unknowingly talks about the boy and says certain private things about his family. There are very private things that she knows about my family which I didn’t tell anyone. The first time she made a comment regarding one of such things, I ignored her but the other day we were discussing it, she still said something private. I got interested and I asked her how come she knew so much about the guy and the private matters of the family. She said nothing, that she was just close to the boy and he told her things. 

She said the guy considers her as family and since she’s my sister he doesn’t see anything wrong in talking to her about the family affairs. I wasn’t buying any of those things she was saying because I could see through the lies and everything. I did my own background search and I found out that they were dating. I asked her and she couldn’t deny it further. She has many of his pictures on her phone including pictures that they took together. The one that even got me crazy was the fact that she travels from school here in Lagos all the way to Abuja to visit him. It is a very stupid thing to do and I don’t know what gave her the effrontery to do that. 

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I saw some of her pictures in his house including videos of both of them even when she was not properly dressed. I don’t know why these two people are misbehaving. They obviously can’t marry each other so why are they even dating in the first place? I have tried to talk sense into her head but she doesn’t seem to understand what I am saying. She just keeps saying she loves him and he loves her in return. I just feel like slapping her. The other grown ass man who should know better is not even helping matters. I want the relationship to end. How do I achieve that?