The actual town of Sewickley has very few, if any septic tanks which I have run into. Nearby Edgeworth only has a handful remaining as well.
We recently had a situation with a "grandfathered-in" septic system where the town had not previously required a connection to city sewer lines. However the septic system failed a test during home inspections, and due to today's regulations, the state DEP would not authorize a new system. In the end, the home owner was forced to connect to city sewers in order to sell/transfer the property as it is very difficult to close a loan on a property with a failed septic system.
If you are referring to the local School District area (Quaker Valley) then your mileage may vary wildly depending on exactly where the property is located. Many of the surrounding towns including Bell Acres, Allepo and Sewickley Hills have been systematically upgrading their infrastructure, often first bringing in water, then later sanitary lines and natural gas. Your best bet is to call the local municipal hall for more information regarding a particular property. Tap in fees are typically the responsibility of the home owner, and can vary wildly - or be flat.