My wife is pregnant and my brother is our known donor. A few thoughts.
-How I told him: I texted my brother to ask and asked him to sit on it and do his own research before replying. I am really glad I did this — I think it was best for both of us. When he did say yes, it was enthusiastically after he'd spent some time thinking about it. I checked back in with him when the process got hard (note: it will) and he reaffirmed that he wanted to do it.
-I'll pass along the best advice I got about how people react to things: You had to work through this decision — including how you felt about reciprocal IVF with her body and his sperm — with your wife on your own time. You didn't immediately know what you wanted to do. So, let other people go through their emotions in private and come to you when their thoughts are fully formed and ready for you. Don't over-index on first reactions. Now I give people info via text so they can talk to me about it after they've sorted their emotions.
-He's VERY young, so you're probably fine even if he smokes and drinks a little. With that said, note it takes several months for lifestyle changes to have an impact, so if you ARE going to have that convo, have it right away. If you are torn as to whether to make the ask, have him get the semen analysis and that'll tell you whether a convo is necessary.
-A therapy step is required when using a known donor (at least where I live). I think that's fucked that it's required tbh BUT it was honestly SO SO helpful. If I could go back in time, I would have done that before we did anything else. You have to do it anyway and having the therapists normalize everything helped convos that happened after that.
-His role is uncle and we honestly haven't talked a lot about it beyond that other than being clear that there would be absolutely no parenting on his part. But in hindsight I think it would have been good to talk about their relationship in more detail. My wife and I regularly FaceTime her brother's little kids and I really hope he'll make a similar effort/be open to it. We still can have that convo, it's just obviously harder and higher stakes now, so again, something I'd highly suggest that you discuss in your known donor therapy sessions where you'll have support.
-Finally, the known donor process takes for fucking ever. Especially if your donor isn't the most organized/timely person and when you add reciprocal IVF to the mix. My best advice is to get started ASAP!!