Appreciate your concern.
At this age normal Hb is 10.3--12.4
Your first twin;s Hb is on lower end of normal range at this age.
Though you have not given which hernia he has ,common hernias
do not cause low Hb.
Only rare hiatus hernia
can cause anaemia
Slightly low Hb per se will not affect his growth & brain development unless he is also iron deficient.
is diagnosed by looking at blood film, red cell indices & ferritin levels.
If you are too anxious ask your pediatrician
about these tests. even if diagnosed iron deficiency is easily treatable.
You are right, the second twin has definitely normal Hb.
Hope I could clear your doubts.