We were told DD (now 5.5) was probably autistic when DS (now nearly 3) was just born (I think he was 10 days old or something). I was just in shock and really my whole maternity leave now I look back on it was a bit of a blur - DS was a much less good sleeper than DD so I was sleep deprived and shocked and worried and just overcome really.
Nurturing Neurodiversity also has a video with her husband which your DH might find useful, where he talks about his reactions to finding out their son is autistic. There is also a channel called Coming Home to Autism which has a very overdramatic titled video (autism definitely did not end their relationship!) but actually it's mainly the husband talking about how he struggled to come to terms with it and then realised he was being an idiot (and she realised she needed to let him in more), so might be worth a watch too! Both of those families have boys who were/are nonverbal so slightly different situations but I think a lot of the emotions you go through are very similar.
Later it might also be worth checking out some autistic women on YouTube to get an adult perspective. I particularly like Purple Ella and Yo Samdy Sam (both also Mums so they get parenting too! Two of Ella's children are also autistic. Sam's child is still a toddler I think, and no mention of their neurology that I've seen!)
I also second all of open's recommendations above. There is also an SLT channel on YouTube called Chirp that has some quite useful stuff on engagement and sensory needs. It's funny when we were first told about DD I thought she had no sensory needs but as I've learned more I've realised she actually has quite a lot of sensory differences (some of them I didn't spot because they were the same as mine: like you I actually find my neurotypical DS harder to parent in some ways than DD - I find it hard to figure out his drama sometimes! Though getting easier now he is a bit more grown up).
It sounds like your DD has a lot of good skills already. My DD also wasn't very interested in social things when she was your DD's age, but over time she has become much more social (still less than her peers, but enough that things like school and parties and play dates work OK).
The key things we've tried to teach her as she got older are just skills so she can express her needs (so if she's not enjoying playing with another kid, we're trying to teach her ways to change to a game they can both enjoy, rather than just wandering off which would be her natural response, and if she's at a party and it's too much and she takes herself into another room or under the table we praise her for taking a break as we find out that works much better and I think it's a good life skill for her to recognise when she's overwhelmed and give herself a break).
The sibling thing is hard. Younger siblings are annoying for any child (especially when they're too young to really be fun or follow instructions: DD and DS get on much better now DS is old enough to play the games that DD comes up with!). I think that's particularly true for our ND kids who tend to be a bit anxious and to need to control their environment and have lots of down time to deal with that. Does DD2 take naps? If so maybe you can use that time to connect more with DD1 (and/or to give her some "me time" when she can play her own way, listen to an audio book or some music, or do things that are tricky when DD2 is around?) Otherwise we tried to just have some protected spaces for DD - so e.g. she would do some arts and crafts high up where DS couldn't reach. We always praised her being a good sister and tried to give her big sister jobs that she would enjoy, and that we could praise her for, so she got a mental impression of herself as a good big sister. We also tried to make things quite explicitly equal for DS: so although sometimes we have to say "well DS can do that because he's younger, and you were also allowed to do that when you were his age", sometimes I would also tell DS off for things that were a bit ridiculous because I wanted to make a point of things being fair. DD is very big on fairness and logic, so I knew if DS seemed to be "getting away with things" she woudl resent it!
She is a really good big sister now so she's kind of grown into the role as she's got bigger and DS has got more interesting!
In terms of telling people I told pretty much everyone when DD was going through the Dx process. I didn't really decide to, I was just so anxious and processing so much and couldn't help myself. I'm not particularly proud of it but I don't think it did any harm, and actually it was amazing how many people had a niece or nephew on the spectrum or knew something about it some other way, so I actually picked up some good knowledge and contacts that way. Maybe I was just lucky but nobody had a bad reaction at all or cut us out. I think actually maybe they cut DD a bit more slack because they knew to expect her to do things differently and that she wasn't being naughty or rude when she did that.
Definitely check out the Nurturing Neurodiversity Facebook group though. Nearly everybody who posts there is still going through the diagnosis process. It's mainly UK based so there's lots of practical knowledge building up there too (it's a new community, only been going a few months).
If your DD likes factual questions have you tried the books "You Choose", "Just Imagine", "You Choose in Space", etc? We used those a lot to start to build conversation skills. I think DD was a bit older than your DD when we did it. To begin with we just asked her questions about the pages ("which pet would you like to have?") and then over time we started modelling that she could ask us the same question back (you feel like an idiot but literally just saying "Which pet would you have Mummy? Oh, I think I'd like the lion with purple stripes"). And then over time you can prompt less (e.g. just say "Which...." and leave a gap ... and then later just leave a gap). Obviously you need them to be ready to do it - but it's still a fun book just to ask questions about when they're not ready to have proper conversations yet.
Good luck, and do come back to ask any questions you think of!