In 5th grade, my kid similarly on the fence and we said yes. ASD was listed as a diagnosis but in the narrative, the provider made it obvious it was on the border.
When things fell apart in middle school, having the ASD diagnosis in paper helped get more support that was key to getting back on track. The autism program at school was also key for my kid finding a group of “kids like me”.
It also ruled out a number of private school options we considered before sticking with public.
In your case, your kid is already familiar with ASD from their sibling, so I would take that into account when considering how it will help their understanding of themselves- especially if the sibling presents completely differently.