Me, sister and her ex

I think my sister is depressed. She had long relationship with her boyfriend, 4 years long but it was actually was a breaking one long time ago. Last two years it was like they were pretending like they were dating, they are so distant. Now he, as her ex likes her as a good friend and wish her the best. He is very crushed that she is depressed like that. Eventually i met her boyfriend, like two and a half years before. We had many same interests, bunch of topics to talk about, basically we became a good friends. Two years ago, they said that they gonna buy an appartmen with two rooms, and i could stay with them (i lived in dorms, barely ate and hardly took care of myself, so they wanted to help). But in reality my sister really didnt said anything and she thought it was a bad idea. But i cant read minds, neither me, or her boyfriend knew anything. And half a year after living together, he developed feelings for me. So do I, but i tried to hide them. A few months after, they nearly broke up and i thought it was my fault. A year after i made my mind and try to fix what i thought i did wrong. I tried to give him some tips, encourage him to show more affection to my sister. But i was making things worse. They eventually broke up. And i thought it was my fault, because i lived with them. But he cleared me up that i was mistaken, their relationship was already going downhill even before i moved in. When they broke up, i also had a huge fight with her ex, but our fight was kinda childish, we memeing it now, that we acted like two thirteen year olds. I, of course, moved out of their appartment into dorms again. Recently we made up. Became friends once again. We sweared to each other not to lie ever again. And it ended up... With a kiss... We sorta started dating secretly. He told my sister, because he didnt want to keep any secrets from her. And she was sad. I know its wrong. And i dont know what to do anymore. She was kinda happy when i had a fight with her ex, when i talked shit about him. And now shes sad (and she was long time ago, three years ago but with time its gets worse and worse) Barely eats. I dont want her to be that depressed but i also care a lot about her ex... i feel so torn apart. What should i do?

Sister's are forever and boyfriends just come and go... You are literally breaking your sister heart. Do you think that boy really deserve's it? First she needs professional help. A lot of things can go wrong with the healing at home only. Trust me... Talk to her. Talk with your parents and explain whats going on, or to anyone that can offer adequate help. Whatever you decide to do, remember that your sister will always be with you and will always remember whatever you as a betrayal. If you really like him, you need to have a talk with her first when she is better and see what she says about it. I dont think this is your fault, but you guys arent also giving her time to heal. Because since you are close to her she will always have him nearby. And when you break with someone the best thing you need is distance.