I've never been diagnosed with anything, but I'm really socially awkward, so I feel like I probably have something!

I've worked in autism diagnosis for a few years. What I can say is:
1- in women it can be really hard to diagnose. Most of the information you can read about autism is more about the masculine form, and there are difference between genders in autism (not all the time of course, I'm not trying to make 2 claerly separate categories). Because of that, women are often diagnosed later, if diagnosed at all.
2- What I've learnt from people that we diagnosed it that often, there is an initial shock of "of shit I have this incurable mental illness" or whatever, but slowly all the things they've been through make sense and after a whil;e, it's more a relief to know the diagnosis. You don't have to act neurotypical anymore (neurotypicals are people without autism), you can learn to be yourself instead of compensating and getting burnt out by what you're trying to compensate.
3- Autism is DEFINITELY NOT an illness, like depression or anxiety. It is a different way to see the world, your brain is wired differently. The problem is that our society is very much made for people without autism: everything has to be fast, we need to be very flexible and being social is seen as positive. Because of that, people with autism can feel very different and get low self esteem, but let's not forget that people with autism usually have a whole set of skills that we neurotypicals cannot have.

So please try to see the whole diagnosis process as a positive development in self knowledge and how to go on from here instead of seeing it as a "there's something wrong with me". I know it's probabley very stressful for your sister and your family, but usually it's not negative at all. But it is very hard to diagnose in normally functionning people, especially women, and it usually takes a whole team of professionals to diagnose.
There's no cure for autism because there's nothing wrong with people with autism.

I didn't imply you were ignorant about the subject, I just thought I'd take the time to share a few things, but okay.

i'm diagnosed for generalized anxiety and SOME form of depression, not sure what yet but it's definitely something. i'm just barely below threshold for a few other things like bipolar and bpd. my brother is autistic with a speech disorder and adhd.

my parents don't exactly believe in mental illness though (chalk it up to asian culture lol) so getting any kind of support when it takes so much out of me just to schedule an appointment is a lot of trouble

i don't really get much choice but to learn to deal with it since i have a huge responsibility to my family to take care of and support my brother as the eldest in the family, but just getting my degree has been a challenge on account of being unable to handle large amounts of stress at a time. thankfully i'm pretty self-aware so i can work through challenges proactively rather than reactionarily.

the rest of my family though... oh boy. they are a piece of work.

I'm autistic and ADHD, but I think the ADHD traits for me, are way more apparent. I'm what people may call "high functioning" autistic, but let me just say right now, those labels (high or low functioning) are bullshit, since many so called high functioning autistic people are good at masking their traits, especially when met with trauma or other situations where they have to try and pass as neurotypical, so using such a label makes others tend to overlook their experiences because "oh, they get on just fine! they don't need help or support!" Meanwhile, "low functioning" labels tend to cause others to view the autistic person as helpless and it leads to a lot of ableism and denial of agency.

I have severe social anxiety and some kind of depression that's exacerbated by traits of my autism/ADHD that I don't receive support or accommodation for. That being said, being autistic is not a bad thing and it doesn't need to be fixed. I feel that you can help your sister and your family by doing more research and informing them, or advising them to ask the doctor(s) more questions about the diagnosis. I totally understand that there's a stigma and loads of misinformation that might make families despair and panic about that kind of diagnosis, so it's important to set the record straight so people don't get mistreated, accidentally or otherwise.