There is no set report on ScholarPack for this.
You could run an Attendance Report for the whole school (either % Attendance Between or the Custom Attendance Report, group by Students), and filter by surname (click in the title column header to filter): Custom Attendance Report
This will only work for siblings with the same surname, as it will place their attendance figures together.
Alternatively you could set up a static group in Data Digger for siblings (names found by running the Siblings Report) and run the % Attendance Between report using this group: Creating a Dynamic Group or Static Group in the Data Digger