The original post, for posterity:

My baby brother Andy [25/M] has never been a whiz with the ladies. He was an extremely awkward boy, a very awkward teenager and only really grew into his body and self probably 4 years ago. Andy was always more interested in academic pursuits, would read French philosophy by the pool and would actively volunteer for summer school to learn new languages. He would spend weeks on end with our elderly neighbors who taught him Russian and Yiddish - we're neither Russian nor Jewish, but my WASP brother stuns elderly Jews with a complete fluency in Yiddish. Andy has a great job, is really well educated and has become a very handsome and funny man. But, I'm very protective. He had a rough time in school and I would have to defend him from guys in his grade. So, I still make sure he's okay.

My friend Alyssa and I met in nursing school and have been really close since, even currently working together at a day surgery unit. Alyssa has known my brother from the tail end of his awkward phase. She's always been courteous with him and they'd chat when they were both together but there was never any inkling of anything between them. My brother keeps his private life very private, I wasn't even sure if he'd ever really had a girlfriend, or lost his virginity. He's not one to talk about it and has never brought anyone home.

So, yesterday we had made plans for everyone to go for an early dinner. My parents were going to a show and me, Andy and my parents arranged to meet at a restaurant not far from Andy's apartment. I sent Andy a text that I was going to drop by and let myself in (I have a key to his place as he has a key to mine). I got there and let myself in. Usually his place is a boy's den. I figured I'd clean a bit but when I arrived it was immaculate. Then I noticed women's shoes and a handbag. I'll admit, my first thought was that he was cross dressing. I pressed further and I could see into his bedroom and while I fortunately couldn't see too much, I got what was going on. I got the hell out of there, quietly locked the door and bolted around the corner.

I sent a text saying I was running late and would meet him the half way mark to the restaurant. He immediately texted back and seemed relieved and said he was on his way. I quickly received another text saying he randomly bumped into Alyssa and would bring her along. We met up and they were laughing about whatnot and we walked to the restaurant. They played it like it was a completely random happenstance and at dinner didn't interact all that much. After dinner, my parents left and we went for dessert. Afterward he offered to walk her home. We all hugged and the two of them left in a full sprint.

When I got home I told my husband what I had seen (he was helping his brother move) and he didn't see a problem with it and thinks that I should just talk to my brother/Alyssa about it if it's bothering me. I'm not sure what to do. Alyssa is my best friend and while I think she's great and would make a great girlfriend, she has a lot more experience than my brother. I feel a bit protective of him. On the other hand, he's growing up and is free to date. From what I can gather he seems to be pretty happy. I arranged to have coffee with him today under the guise that my husband is still helping his brother and I was bored and in the area. We're meeting for coffee in a little while and I'm really not sure if I should say anything or keep my mouth shut.

Would it be a gross violation to tell him what happened? My big thing is that I want my brother to be happy but I also worry about him and want what's best for him. Can I bring it up and talk to him about everything?

Edit: Adding this comment to better explain Alyssa:

She's not terrible, I'm wording it wrong. Alyssa confided that she's "done" with dating and looking to "lock things down." Alyssa is pretty enough and wily enough that she could close that kind of deal with anyone. She'd be a great wife and a great mother, but she could lock it down before he really thinks about it. She's really sick of being single and my brother has certainly never met a tour de force like Alyssa before. She'll hit like a tornado and rearrange his life in seconds. My brother is an absolute catch - he's very well educated, thoughtful, has a great job (though he's really just starting out) and is good with money. She wants a husband/family/stable guy and I could easily see where this is going. She and I have been friends for a long time (~7 years) and we talk every single day. I know what she's looking for. And, what is there for her not to like? I can guarantee you that they could easily be engaged in weeks.

tl;dr: I let myself into my brother's apartment (I texted first) and got an eye full of him with my best friend. Neither know what I saw and I'm not sure if I should let him know what I know or if I should try and keep it quiet. Really not sure how to proceed.