AITA for calling out my sister after she started dating my ex?

I (25F) have a sister, Lily (23F), who I’ve always been close with. We’ve been through a lot together, and I’ve always been there for her, especially during tough times. We’ve had our arguments, like all siblings, but I’ve always thought we were each other's biggest supporters.

About a year ago, I broke up with my long-term boyfriend, Jake (26M). We had been together for four years, and it was a difficult breakup. There was no cheating or anything malicious—it was just one of those relationships where we had grown apart, and our paths were no longer aligning. I was heartbroken at the time, but we ended things amicably, or so I thought.

A few months after the breakup, I started noticing something odd. Lily and Jake started spending more time together—initially, I didn’t think much of it. They’d run into each other at family gatherings or go out for coffee, which seemed normal, considering they had been friends before we started dating.

But then, I started feeling like something was off. I noticed Jake texting Lily more often, and at family events, they’d act too friendly. I didn’t want to jump to conclusions, so I ignored it.

Then, last week, I walked into my sister’s apartment unannounced. I know that might seem a bit rude, but we’ve always had an open-door policy. When I walked in, I saw them sitting on the couch together, laughing and talking. But what really caught me off guard was the way they were looking at each other. It was the look—the same way Jake used to look at me when we were together.

I froze. My heart sank.

“Are you two…?” I asked, but I couldn’t finish my sentence.

Lily immediately went red, and Jake shifted uncomfortably. They both avoided eye contact, but I could tell exactly what was happening. They had been seeing each other in secret for a while.

I didn’t know how to react. I felt betrayed by both of them. Lily, my sister, the person I thought I could trust the most, had been dating the guy I had just broken up with—not even a year ago. And Jake, who I had loved, had started a relationship with my sister without even telling me.

I called them both out. I told Lily that it was a huge betrayal, and she should have respected me enough to never cross that line. She apologized, saying that she didn’t want to hurt me, but she had developed feelings for him after the breakup. She didn’t think it would bother me so much, but I felt completely blindsided.

Jake tried to defend himself, saying he never meant to hurt me and that they both wanted to tell me sooner but didn’t know how. Honestly, I didn’t care about their excuses. I felt like they had both betrayed me in a way I couldn’t forgive.

After the confrontation, things have been tense between us. Lily is upset that I’m not more understanding, and Jake has been avoiding me. My family is torn—some think I’m overreacting and that I should be happy for them if they’ve found love. Others agree with me, saying that it’s wrong for them to start a relationship without considering my feelings.

So, AITA for calling out my sister and ex for secretly dating, or am I just being too possessive and need to let them live their lives?