I don't think that personality is completely hard-wired, but different people will strongly tend toward different traits, and two people raised in an identical environment will react differently to that environment.
Interests can be encouraged because there's some flexibility in someone's natural tendencies. It's the old "nature vs nurture" discussion, about which one dominates the other.
Not to mention that siblings by definition each grow up in a different environment. The addition of a new sibling changes the behaviour of parents and other siblings (sometimes a lot, sometimes a little, but it's there).
Perhaps the above poster's sibling looked for other interests because the poster had already 'taken' whatever interests they were sharing. But perhaps not - it's hard (impossible?) to know.
Interests can be encouraged because there's some flexibility in someone's natural tendencies. It's the old "nature vs nurture" discussion, about which one dominates the other.