I am a fan of Clive Cussler's Dirk Pitt books....?

Dirk was banging Summer Moran in Pacific Vortex (an older novel). At the end, she was supposedly crushed in (I think) an avalanche of rocks. Lo and behold! Her 2 kids show up at Dirk's airplane hangar at the end of a later book. It was a lame way to bring them into the series.

Like the other answer, the books became stupid when Clive wrote himself into the story. Especially one where the heroes were in the middle of the Pacific Ocean and Clive just happened to be boating in the middle of the night and came across them. Of course, Clive let the team borrow his diving equipment to get into the villain's hideout. That's my own personal rant.

Then Cussler just duplicates Dirk & Al with Kurt Austin & Joe Zavala in NUMA file stories.

The first they appeared to me was in Treasure of Kahn. I may have missed one or two of the Pitt novels when Cussler started writing them again when his others weren't such a hit. There was some backstory about the mother dying but I don't really remember. I started burning out on the Pitt series when Cussler insisted on writing himself into a scene. I thought it kind of silly. I think they appeared before Treasure.