Are friends' brothers off limits? / situations with my friend's brother?

my friend and coworker (who is 2 years younger than me) has an brother who is 2 years older than me, and he is very attractive. he also works at the same place we do, but I don't see either of them often. I'm not super close with his sister, but I talk to her a couple times a week and I really like her, so I definitely would not want to lose her as a friend. however, I am attracted to her brother and I'm thinking there's a chance he's a bit interested in me?

when I was talking to her one day, a few weeks after we all started work, she told me that her brother asked about me, saying that he hadn't seen me in a long time (the main reason we know each other is because of mutual friends). a few weeks later, I saw him when I walked into work and texted his sister observing that he was very tall and she responded by saying "at least you didn't say he was hot".. apparently she gets that a lot. I told her that I didn't even see his face (jokingly) and she sent me a video of him, which was weird so I responded with jokes, sheerly because I didn't know how to respond...

fast forward to 3 days ago, I walked into work again and he was near the place where we sign in. we made eye contact as I was walking closer and he raised his eyebrows at first and then waved, but because I'm oblivious, it took me a while to realize it so I kind of half-smiled and waved a bit.. when i walked over he stood up and started talking to another one of our coworkers, so I didn't get the chance to talk to him.

anyway, I have 2 questions:
1. is a friend's sibling always off limits?
2. can someone try to interpret parts of these situations, especially the last one I described?

I'm so sorry this was so long, I just wanted to try and give some background. thanks!!
Are friends' brothers off limits? / situations with my friend's brother?
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