Hi @james_ahb welcome to the family hub. You are not alone. I’m a sibling too (my brother has severe learning disabilities) and there are many other siblings out there who have been through the diagnosis period over several years, just as you are doing now.
It sounds like an incredibly difficult time - for your brother, your parents and for you too. You care so much about your brother, you just want him to be safe, happy, healthy and have the support he needs. You want him to thrive and have the same life and opportunities that you do. Sitting there, not knowing what is going on, why, or when you will have some answers is excruciating. Of course, you want to ‘do’ something, you want to be active, you want to help him in any way you can. The waiting is so hard and I don’t know a way to make diagnosis any quicker. Keep doing what you are doing - being your brother’s brother. Just being there for him, just having the relationship that you do (whatever shape or form that takes) is already meaningful and helpful. Make sure you get some support for yourself too, for the worry and anxiety. Whether that’s getting in touch with Sibs (see below), meeting other siblings, talking to a friend, or seeing a counsellor if it helps. When you’re supported, you can support your brother too.
Take care and keep in touch
Sibs - the UK charity for brothers and sisters of disabled children and adults
Contact - https://www.sibs.org.uk/contact/
Join your nearest support group and meet others - https://www.sibs.org.uk/support-for-adult-siblings/adult-sibling-support-groups/
Website - www.sibs.org.uk
twitter - @sibs_uk
facebook - @sibscharity
SWAN UK (who support families affected by a Syndrome Without A Name).
Website - www.undiagnosed.org.uk
twitter - @swan_uk
facebook - @SwanchildrenUK