They are saying that the contract or document or the insurance coverage provided applies to brothers and sisters (over the age of 18) that are permanently living with you.
They are using "(over the age of 18)" to separate them from brothers and sisters under the age of 18.
In other words - whatever the contract or document says related to that sentence only applies to brothers and sisters over the age of 18. It does not apply to brothers and sisters under the age of 18.
- For Teachers