My second biggest sister

[Xinfu]

John has two sisters his senior, Mary and Betty. Mary is older than Betty.

How should John refer to them with respect to their positions to him?

-Mary is John's biggest sister.
-Mary is my biggest sister.

-Betty is John's second sister.
-Betty is my second sister.

[Rusty]

The word 'second' in your last two sentences needs to be followed with 'biggest', like it is in your thread title. Then all is correct.

[Xinfu]

Thank you~