My wife and I come from two loving families - reasonably sized - I've 3 brothers and she has a brother and sister - both sets of our parents are in their 70's now. Neither of our families have had experience of a diagnosis of ASD or ADHD. Her family are quite local to us and her parents have been a God send to us, hugely supportive of us, very patient and always at the end of the phone or ready to pop up when needed.
My own family are based 100 miles away and whilst they love the grandchildren dearly, visits and involvement is really only at the holidays. My mother has a degenerative brain disease, so we have to be very careful in managing visits with the kids when we go and visit.
We both grew up in the 70's and 80's where a clip round the ear, a wooden spoon or whatever came to hand first was the rule at home. At school the black strap across the hands (6 of the best) was how kids were disciplined. Thankfully those days are gone, but as our eldest was 'stepping out of line' it's what we were told might be missing (from both sets of grandparents). Their attitude changed with diagnosis but that took almost 9 years. We never took the advice, thankfully. I can't say I wasn't tempted, if I thought it would solve something rather than make a difficult situation worse.
My wife's siblings work in education, as does one of their partners. They had reasonable interaction levels with our lad (from birth until COVID) and only once did they say to us that our lad's behavior might be 'off' for something beyond a cheeky brat. They do work in secondary schools though.
My father in law and mother in law holiday with us each Summer for a week or two. They've been hugely involved in our boys lives. We even lived with them for an extended period. We've been truly blessed to have their support. They have a fantastic relationship with both our boys and more often than not, the boys first port of call if anything is bothering them. It's not all been roses but they have been more than good.
My father in law, has a way of doing things and getting things done, that makes sense. If you don't do it his way, there's something wrong with that. I've seen him wash dishes immediately after plating food, as it's easier to wash the pots and pans then, than do it after you've eaten. His sister (my wife's aunt) worked in medical science laboratory, as does my wife. Sticklers for details and detailed process.
One of my brothers, we called him the runt growing up, if there was ever anything medically to go wrong in our family, it was to him. I go through these loved individuals and highlight some quirks because every family is a family. A group of individuals brought together through love, the accident of birth and everyone has their own quirks in the group.
When, as parents, the child professionals/specialists talk to you or interview you as you begin your journey towards a diagnosis, they will ask about your family history. Show me a family without quirks and I'll show you a cardboard cut out or a photo frame shot. ASD is no-ones fault. Not your parents. Not your extended family's. Not yours and certainly not your child's!
At this stage of early possible diagnosis, it can feel like there is a fault, blame or source/trigger. It's not the case.
All you need from your extended family is a bit of understanding and patience. Especially before or as you go through the process of diagnosis, whatever the outcome. Once the family have a label, they will understand and come around in their own fashion. You may need to work with them a little to help keep their support and understanding but at the end of the day family is family.
Our families were the first people we told once we had a diagnosis. We took a few days for us to absorb the potentials of the future. A few days for us to breathe, in the knowledge that we weren't 'bad parents'. A few days for us think about how, what and whom to tell. My wife's family knew we were going through the process of diagnosis. My own, with being so far away and not having such a regular connection with our lad, I felt it wasn't appropriate to tell them of the process. It was purely an individual choice on my behalf. Both families were very supportive, picked up a few pieces of information on ADHD and ASD so they could understand further as well - we helped by directing them towards selected YouTube videos/webinars etc.
When it come to getting a diagnosis, who you tell, what you tell and how you tell is your business. Not anyone else's. Some will push for information. Others will let you inform them as you are ready. If you choose not to tell anyone, for whatever reason or choice, it's no one else's business. What we found beneficial was being able to provide some information for people we told to digest a little. I definitely wouldn't suggest unleashing them on YouTube searching ASD - you never know what horror story they might find. So don't give any information to someone unless it's the information you want to give.