Actually this is from one of his previous works the "Tatoeba Haha Ga Volumes" it is between Vol. 1 and Vol. 2 before the son finds out about the relationship of his best friend and his mother. The only reason she is called aunt is because in the volumes he calls her Obaasan, which is a friendly term for nonrelation people in situations of younger man and older woman.
This really makes a good argument for not translating honorifics. Using obasan and having the few who don't know what it means actually learn something is better than using an overly literal translation that can confuse anyone who reads.