Also, this 'best practice' plan is going to take a loooooong time to work, and it may require you to calmly restate the rule/your boundary every single time. Literally every time. It sucks, but it's not like your autistic relative is doing it on purpose.

You are allowed! By enforcing reasonable social boundaries, you'll be doing this person long-term good.

Other parent definitely needs to help you with this; talk to them and get them on board first. Then, best practice would be for you and Other Parent to explain to this kid/young adult at a time when you are all calm. By which I mean both that *you* are "all calm" and that everyone is calm. Use simple direct statements. Explain that you sometimes need alone time when no on can talk to you, and you can't always just switch gears right away when someone else wants to chat. Then plan/agree on definite blackout times and some signal that everyone knows for other blackouts; make sure that you use that signal; and when your stepkid misses it, calmly and firmly say, "I need some alone time right now. Can we talk about this later?"** (It may help to designate an alone-time or non-chatting place. "I'm in my bedroom now. No chatting in the bedroom." "I'm at cooking right now and need to concentrate. Let's put this on the list to talk about after dinner." )

Of course if you offer to listen later, then you have to give them the chance to talk about it later, but at least you can be more prepared. In theory we have a 'list for later' but in practice we mostly don't use it, or we put topics on and then forget about them, etc. Offering to listen later definitely softens the blow for both my diagnosed autist and my quirky not-quite-NT kid.

It will also help if you can actually be an active (or enthusiastic) listener at other times. Over time, we have half-consciously and half-unconsciously trained our kids to talk to different parents about different topics. I get the social/anthropology questions and video game narrations; Dad gets the math questions and the hypothetical board game scenarios.

Getting personal: I am not a confident or natural cook and I find the sound of our stove exhaust fan immensely grating. We have family rules of 1) NO ONE TALK TO ME WHILE I AM COOKING, 2) everyone in the family has the right to refuse to talk to others when they're in their bedrooms, and 3) don't talk to me if I am wearing my bright yellow hearing-protector earmuffs.

** Instead of "Can we talk about it later," you may want to go more declarative, "We can talk about this later," or even specifying when you are willing to talk about it, as, "We can talk about this after dinner."

All you had to do was leave out the -ball. "I am a transparent eye" would work just fine! It still wouldn't be especially subtle or anything, but at least it wouldn't be a disembodied organ.

Sluggish cognitive tempo is not the most flattering name, but I'd be willing to claim it, especially if it involves some executive dysfunction. I transition slowly, process slowly, often need to have a good night's sleep in order to really 'get' something. The last one I know because it drove my first manager (fast food) crazy that I could mess up the same simple thing all day but come in the next day and do it right; why couldn't I just do it right after five or ten tries like everyone else?! Dude, I don't know, but we both see the same thing happening, and it's not like I enjoy getting things wrong. It was actually really helpful to me to have that insight when I was 16, though never quite as clearly and directly demonstrated since then.

Every day I am glad that I live in the age of the internet; asynchronous text-based communication is basically what allows me to have multiple friends and actually be a friend to people. Plus, online/automatic bill pay.

I think/hope there has been a big shift in the ten(!) years since my daughter was diagnosed, when most of the books recommended to parents were about the 'core deficits' of people with autism and how to correct them, and I hope this book continues that shift.