My (35F) brother (17M) is dating my ex's (22F) little sister (18F) - how do I get over this?

My brother is very important to me, and is a wonderful kid. I would do anything for him. My ex, Rachel, is someone I dated for three years and broke up with two years ago. Rachel is not as important, but her sister is one of her three siblings, and I have never met her. It would never have occurred to me to date her, because I am not ready to date much younger, nor do I want to try to date a sibling of my ex. I have always thought of her as a kid, more or less.<br><br>I have been single for a while and am just starting to take an interest in people again. I have a two potential women I am interested in who have expressed mutual interest in me - one from my work, and one that I met through a friend. I am not in a rush to make a choice, as I am just getting to know both. Before I can get into a new relationship, I want to make sure I am over my last one, as I feel like I can see enough red flags in advance to avoid a rebound relationship. But two things happened simultaneously that have thrown me into a bit of a tailspin.<br><br>First, I ran into Rachel somewhere, and she asked me out for coffee. I haven't seen her in a long time, but I still had some feelings for her, and we hadn't parted on bad terms, so I said sure. We talked for a while, and it became clear that she hadn't moved on from me, and neither had I from her. We ended up back at my place, and ended up in bed together. A couple more dates like that, and I went over to her place, and discovered that she has a new sister. That is, a couple of years ago, she adopted a kid, and now the kid is basically an adult. Anyway, I met her, let's call her "Jessie". Jessie is beautiful, and I started to feel something for her almost right away. I think it was just physical attraction, at least it is now, but I liked her very much.<br><br>Then this weekend, I went to see my brother, and met his new girlfriend. It was Jessie. She is 18, he is 17. They are very much in love and very cute together - they are going to a school dance on Saturday. I don't get the feeling that this is a fling or a hookup, they are in a relationship, and I am happy for my brother that he has found someone he cares for.<br><br>I want to be happy for my brother, but I'm not. I feel like I got blindsided. First, my ex is still in love with me, and I still love her. We could have gotten back together. But now she is off the market. I may get over this, but while I was dating her again, I met someone else that I started to feel something for. But that person has chosen my brother over me. I don't know if she ever felt anything for me, or if I just imagined it, but I don't see her in a romantic way anymore because of my bother. In the same week, I lost two potential romantic partners, and I was never even in a relationship with either.<br><br>How do I handle this? I have a lot of years invested in these two individuals, and it feels like my bother is a bad luck charm. He is a kid and doesn't know any of this, and it isn't his fault, but I am having a hard time getting over it and I need some advice.