On the student profile front page there is a Siblings area to the bottom left of the screen.
Click on the cog next to Siblings and a window will pop up to enable to you link a sibling:
You may find the student under Possible Siblings, if so click on the name and it will move to Currently Assigned Siblings. If the name isn't there, enter the name in the Sibling Search box. If a mistake is made with linking a sibling, click on the name under Currently Assigned Siblings and it will unlink from the student profile.