They're bent on a splice next spring.
bent on a splice
idiomAbout to be married.
Origin
disputed origin* A splice is used to join two ropes to make one. * Possibly a pun. See bent on and bend.
After ten years together, my sister is finally bent on a splice and planning the wedding.
The old sailor, bent on a splice with the harbourmaster's daughter, spent his last coin on a modest ring.