Childrens (i think) book where the main characters brother has an illness that makes him grow too fast

[Isabel]

I believe it was set in late 1960s because i remeber then talking about the moon landing. Her and her brother really liked animals and insects and they read these books about them. Her brother has an illness where he is really tall but hes only a young kid. The cover was brown and had animals on it.

[Isabel]

i read it about 2/3 years ago

[Joseph]

Probably Lenny's Book of Everything, by Karen Foxlee.