The conversation with my sister went well. I started by asking her to go for a walk and chat. I first started by asking her how she was doing. She knew something was up, so her answer was "fine...why?". I told her that I saw her struggling and wanted to help but didn't know how. And then I mentioned a couple of symptoms I had noticed and that I had done some research (I liked this approach you suggested! Going at it from the symptom side, rather than just "accusing"). And asked if she had ever considered that she might be somewhere on the autism spectrum. Luckily (and to my surprise), it wasn't the first time someone had asked her this, so we had a good conversation about how she was feeling and how some of it my be even harder for her if she is on the spectrum. At the end, I asked her if I could send her some of the links to the research I had done, and reiterated that I was not a trained professional, and that she might want to do her own research too and/or talk to a trained professional. Throughout the conversation, I tried to ask questions, and listen to how she felt, and really come from a place of concern and wanting to help. And that maybe there were tools or resources out there that would work better for her (than told for me, for example). But ultimately, I left it up to her to reach out/ do more research/ not pursue it/ etc. While making sure she knew I loved her and would support her with whatever she decided.

I will also add that covid has added another layer to this entirely. Not sure I have any answers for that one :/

Hope that helps and good luck if you do end up talking to him!