Hello Etoile72
The seat reservation for the PE Luzern Interlaken was recently made available through SBB. The seat reservation system is not yet as sophisticated as it should be.
The train is not operated by SBB yet by its subsidiary, Zentralbahn.
For your specific request, I suggest to use the train operator’s seat reservation system which you can find here; it comes with a seat map:
https://store.luzern.com/luzern_en/specials/daytrips/zentralbahn-int-lu/?___from_store=luzern_de
Let us know how it worked for you.
And, small insider tip: take seats at the right hand side in travel direction. It is the side offering better views.
Kind regards,
Roland