elder/older

English as a Second Language English Vocabulary, Grammar and Idioms
1
  1. His elder/older brother serves in the army.
  2. My elder/older brother is a writer.
  3. His eldest/oldest son is in the Army.
  4. He is the oldest/eldest member of the club.
    In each sentence which option is correct?
    Please comment.
    Thanks.
2

1, 2 - all are possible.

3 4 ‘oldest’ is correct.