I didn't even know he had siblings
In his book, he’s the middle one of three boys. His older brother was diagnosed with ‘differently presenting’ (Jase’s description) autism when Jason was very young (ie a couple of years before his own diagnosis/‘diagnosis’), and his younger brother was a ‘prodigy’ because he could read and write at 4. Both have done well education/career-wise, the older in education, the younger in healthcare.
Book tales of Jason’s one to one interaction with his older bro are pretty much limited to him having to take Jason to/from secondary school to make sure he got on the right bus (the 77). There’s nothing as an adult.
The younger one is a convenient supporting character as an adult a couple of times, always where they eat junk food and have convenient plot-advancing conversations while performing DIY tasks at Giff’s house. Eg when Jason was 27, bro encouraging him to write down where they wanted to be in ten years and when Jase wrote down that he wanted to be an Oxbridge professor, bro telling him “if anybody can do it you can, you operate on a higher plane to the rest of us” or some similar
you’re so special and unique and amazing bollox not rooted in anything factual/with any basis in reality whatsoever.