Search Results: letting me know that he has a girlfriend

I live in the US but him and his family lives in Germany. We met because he has a relative who lives close me. My boyfriend has a younger sister who claims to be shy and it was understandable at first, especially due to the language barrier. When I met her, she was 12 and I, 20. Fast forward to last year, I 23 and her 15; I go live with them in Germany to learn the language while I took a gap year from college due to covid. As time progresses and my understanding of the language picks up, I start to see a different side from her. She doesn't acknowledge me, talk to me, or even greet me in the morning. She only refers me as her brother's girlfriend or "sie" which translates to "she" in German. The only times where I've heard her say my name is when she has an accusation to say about me. She's accused me for entering her room, on many occasions. She's accused me of taking her stuff as well. She also claims to not want to try to speak with me because she's too "shy" and that I will laugh at her English. In reality, she's laughed at my German twice (once, infront of her friends and another time during dinner). The tension with her and I just kept getting worse, with bigger claims such as my boyfriend and I letting her starve. Why would it be my responsibility to make sure that a 15 year old ate dinner before going to bed? Anyways 10 days before I left back for the US, her and I got into an argument. I was alone in the house with her and her friend (which gave her the boost of energy to rip me a good one). I left the country on bad terms with her, and not once has she apologized to me for anything in the past 4 years. Now as I said, some of my belongings disappeared while in Germany and I'm discovering that she took them. The only proof I have are my receipts and that she posted a photo online wearing one of my missing dresses. I don't know German law, but she has had a history of shoplifting. I also learned that she is hoarding my stuff at that same friend's house.