@Cantaloupes
seems i am not very good to deliver it in english,
but what it means here is that he and his sister are twins (this might be rare but possible)
so, he (the twin brother) act as substitute of his sister (the twin sister)
sorry if it's weird
now that i look again at the specific page,
i realized my mistake that put 'younger brother'
instead of 'younger sister'
my apologize for this mistake
will fix it ASAP