Was I unreasonable?

I have a sister who is sixteen years old. Her mom left when she was four, leaving her with four very rowdy brothers, and a father, who is bi-polar. Loving, and dedicated, but bi-polar. I have been as much of a mother figure to her as I can be, as I have tried to be with all six of my siblings. Yesterday, she, my oldest brother (25) and my dad came over for a few hours after dinner. As they were leaving, my sib, we'll call her SibXX16, stretched her jacket went up a little and I saw something on the lower right part of her stomach, right above her jeans. At first I thought it might be frayed threads from her jeans, so reached over to pull her jacket up to see. She pulled away from me, so I knew my suspicion was correct.
"Come on upstairs," I said, "I need to show you something in my room." She followed me obediently, and sheepishly.
"Okay," I said, "Let's see it."
She reluctantly lifted her jacket to show me a small graceful treble clef inked on to her lower abdomen.
"When?"
"A couple of months ago."
"With who?"
I already knew the answer, but I needed her to say it. I knew it was her boyfriend of about a year and a half. By all evidence he is a perfectly nice boy from a good, close knit family, but I don't like her being in a serious relationship at such a young age.
"Are you mad?" She asked in a very small voice. This is a tough girl who for all intents and purposes raised herself, so it meant something that she cared if I was mad.
I sighed.
"No, mad is not the right word."
"Disappointed?"
"Yeah, disappointed. I wish you had talked to me first. I would have tried to talk you out of it. I wish that this tattoo was not a link to your boyfriend, but it could be a lot worse."
"Really?"
"Yes, at least it is small and well done, and you know, not your boyfriends name."
She hugged me and thanked me for not freaking out and left my room. As we left my room I made another comment about it not realizing that XX13 was coming out of her room.
"Oh, my mom found out?" She said to SibXX16
"What?! How long have you known?" I spun around and asked. She immediatly realized her mistake.
"Not long," she said evasively and scurried down the stairs. I was on her heels repeating the question. We got down stairs and stopped talking about it because my dad was there. As soon as they all left I started grilling XX13 again.
XY17 was in the kitchen and said,
"Oh mom found, oh... never mind." He went back to the food he was preparing five seconds too late.
"You knew too?!"
"Crap, the second the words were out of my mouth," he muttered looking down at the cheese.
All of the reason and calm that I had shown with my sister was gone. I was practically yelling at them. I was so upset that they didn't tell me.
"We found out the same way you did," they both told me.
"It's not like it was something dangerous," XX13 said, "And besides, she's my aunt, it would be disloyal."
I was irate. I left the room for a minute. Almost instantly I felt like I was being unreasonable. Did I really expect them to come running to me to tattle on SibXX16 for getting a tattoo? They are both very close to her. They all grew up together. I have always encouraged the loyalty and closeness XY17 and XX13 share. I know they know things about each other that Roser and I don't know about them, so wouldn't it make sense that they would extend this loyalty to their aunt, who is really more like a close cousin?
The reason I was upset was because I felt that if they could keep this, pretty big, thing they could keep other things from me. The other reason is because we have recently had a problem with XX13 keeping important information about one of her friends from me, so I saw this as part of a pattern. She tried to assure me this was not the case.
"If it was a friend, I would have told you. Haven't I been honest, even about the bad stuff lately?" I had to admit she had.
The clincher is, I did not, and have no intention of, telling my dad. I am not sure exactly why. There are many reasons, and maybe none of them on their own are a good enough reason not to tell him, but all together, they make me feel justified in not telling him.
Now I am asking you. Was I wrong to get upset? There is a poll on the upper right corner of my blog. Please tell me what you think.