Using "oldest" for younger siblings

The eldest is the oldest of all the children, regardless of the birth order of other siblings. That is what is understood. However, you could create a sentence with clarification that would contradict this.

So, you are 25 years old. How old is the oldest of your younger siblings?

If you don't explain that, the person would take "eldest" to be the oldest of all the siblings.

Thanks a lot for your help! I'm going to try to clarify it a bit. The situation is this:

Sam (26), Bobby (25), Sally (23), Louise (20), Betty (15).

Can I say "This books belonged to Bobby's eldest sister"? I mean the books were Sally's, but she's not the eldest of all the siblings, only the eldest of the sisters. Is it right to say so, or does that sentence imply that the sister

These books belonged to Bobby's eldest sister.

I would not say that because it is very confusing.

These books belonged to the eldest sister in Bobby's family.

Thanks! So it's confusing, as I suspected. I'll avoid it.

These books belonged to Bobby's oldest sister. (the oldest of Bobby's sisters)


These books belonged to Bobby's older sister. (the sister is older than Bobby)