Siem signed a six-year deal at St James' Park on Tuesday to boost Alan Pardew's attacking options, with Newcastle having lost on-loan top scorer Loic Remy.
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However, Louis Laros, the brothers' agent, believes the duo could link up next term as Luuk returned to pre-season training with Borussia Monchengladbach.
"Luuk's very happy for Siem – he gave him good advice," Laros told The Shields Gazette. "The two brothers are not twins but they are very, very close.
"It could be an option for them to play together for Newcastle because the two boys are very complementary to each other.
"Luuk is a striker and Siem plays behind him so it could be the perfect match.
"There's nothing happening at the moment but you never know in the future.At the moment, Luuk is back in Germany, because Newcastle didn't take up the option to sign him.
"Newcastle are still looking for strikers, though, and maybe will come back later in the window for him.
"The door is never closed."
Siem de Jong was Pardew's third summer signing following the capture of striker Ayoze Perez from CD Tenerife and Jack Colback from Sunderland, and it's thought the North East outfit are weighing up a fourth summer move, with efforts stepped up to secure the capture Manchester City's Micah Richards.