You can have both & that's not uncommon.

No, not really. Autism is pretty distinctive, there's far more signs to spot than being quiet and awkward.

I'm not trans but I have a good few friends who are, in various stages of transition. They all describe it the same way: "oh you'll know if you're dysphoric".

Many trans people are autistic, and some of the symptoms can look similar at the surface level, but they are separate and totally possible to be comorbid.

I mean, you can absolutely feel freakin' nothing for you "community" and still be a member. Look at log cabin republicans for example. Or uncle toms...just saying. All things aside, I believe that a lot of people's stances on things are a less intricate glance at someone's inner world. So your sister might not feel that compassionate about her own being. But I also think, and this is a personal opinion, that people nowadays are touting opinions/emotions like they're facts. That people are actually being a bit too overly emotional, and not finding the balance between the two. This coming from an emotional freakin' maelstrom. It's all about balance. So don't rag on your sister for feeling the way she does. If you've got issues, talk to her about them. Try to do it in a mindful way so you don't bulldoze her. If she doesn't want to talk about it - drop it. You know your sister, reach out in the way that's best for her and go through that. Or leave it alone, because people are allowed to feel any which way they want as long as they're not smashing the snikes bikes out of others. S'all.