Been out for 10 years, but have never told my brother. Help!

So to give you some back story, I'm a 26yo bi guy from a "traditional" (aka conservative) midwestern family. I discovered pretty early on that I was interested in both sexes and was pretty comfortable in my bisexuality by the age of 14 or 15. I came out to my parents and older sister when I was 16, which was actually a lot better than I imagined it going. When I told my sister she was just annoyed that I had wasted her time. "Who cares GearlyBeloved? Just get out of my room!" I was mostly nervous about my parents. I asked them if I could "talk to them about something" and was clearly upset and nervous about telling them whatever I was about to tell them. They both noticed my nervousness and looked very concerned, preparing for the worst. When I told them, my dad just shrugged and looked confused. It was the best reaction I could have gotten. "Oh, that's it? I thought you were in trouble!" My mom, however, was less thrilled. As the words "I'm bi" came out of my mouth, she let out an audible "EUGH!", which wasn't a great feeling. We talked it out, they both said they loved me and that this wouldn't change anything, hugs were shared, tears were shed (by me). It was a huge relief, originally thinking I would be disowned. For the most part, they are the same supportive and loving parents I've always known, but they have implemented an unspoken "don't ask don't tell" policy on the subject ever since. Though they ask my older sister every Thanksgiving if she's dating anyone, they've both generally stopped asking me (Dad sometimes will if my mom isn't in the room). They don't want to hear about it, so I'm not going to volunteer my personal life if they don't ask. My parents have both met boyfriends and girlfriends when they've visited me (I now live in Florida) and have been genuinely welcoming and loving. It's a little more awkward with the boyfriends, but after they get to know him, they always have nothing but nice things to say. It's not an ideal situation, but it's a lot better than most people would have, especially for someone coming from my small town in the middle of the country. Not perfect, but it works.Probably the worst part of my coming out in high school was my mom's rule that I could not tell my little brother. He was 8 years old at the time and she was afraid of telling him at such a young age. I was still thrilled to be living under their roof, so I went along with her wishes. I now wish I hadn't. As I look at me and my brother's relationship, it's definitely not as close as I wish it were. My sister and I are incredibly close, talking on the phone almost daily. I really want the same thing with my brother but have so far been unable to talk to him on a personal level. I recently had a rough breakup with a longterm boyfriend that he knew nothing about. My sister was there for me, but I would have loved to have my brother in the mix as well.Which brings me to my dilemma. I've been out to all my close friends, family, and loved ones for a decade now; everyone but my brother. I'm getting sick of this situation and I will be seeing him in about a month for his high school graduation, so I plan to tell him then. I know he won't have a bad reaction to my bisexuality at all (this kid has more gay friends than straight friends), but I am afraid of what he'll think when he finds out that everyone around him knew for years and didn't tell him. I don't want to lie about why I haven't told him, but I also don't want to throw my mom or my family under the bus. I also feel pretty guilty myself for not telling him sooner - I've had plenty of opportunities to tell him, but have always put it off because of how awkward I think it will be after so much time, which obviously only makes the problem bigger. I am 100% going to tell him when I see him in May. Anyone have any advice?TL;DR : Came out to conservative parents 10 years ago and it went better than expected, but not perfect. They wouldn't let me tell my 8yo little brother, who is now 18 and still doesn't know. This has strained me and my brother's relationship and I want to tell him when I see him in a month, but don't know how to do so without hurting his feelings that he hasn't been told earlier. Don't want to throw my family under the bus either.