Please go to Reporting>Reports>Students>Siblings - Run this for all years. Once the report has loaded under the header called "Position in School" type "Eldest" and click enter on your keyboard - this will then filter the report to all your eldest siblings. If you wish to know those who have no siblings in school, enter Solo to filter the report.
If you wish to run the report to find how many households are in the school (for sending a letter to each one), enter Solo or Eldest in the "Position in School" column search box, and ensure you have ticked Show Student Addresses prior to running the report. This way, the report will give you one student per household to receive the school letter.