SOLVED. [Children/YA] Main character has a feral brother

[Peroikos]

Children/young-adult book, read probably 15-20 years ago, but was likely older than that

The only thing I remember about the book is that the little brother starts living under the front porch, and then as the book goes on he is seen less frequently and is found to have expanded into a network of tunnels and gone completely feral. It may have been in response to some trauma to the family in the beginning of the book, but I can't remember anything else.

[Peroikos]

wanttoplayball on reddit remembered for me - it's Thursday's Child by Sonya Hartnett

Thursday's Child

https://www.reddit.com/r/whatsthatboo...