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Selena Gomez 's grandfather proudly gave her away during her fairytale wedding to Benny Blanco on Saturday ... and we're told the decision was a no-brainer.

A family source tells TMZ ... Selena chose her maternal grandfather, David , to walk her down the aisle because she's extremely close to him, and he never got a chance to walk his own daughter, Mandy , down the aisle because she eloped. Getty

Our sources add ... her stepfather Brian has his and Mandy's daughter Gracie to eventually walk down the aisle. We're told by an additional source Selena is quite close to Brian .

Furthermore, our family source says Selena's mom thought having her grandfather give her away was a "thoughtful gesture." Getty

Mandy, who co-founded Wondermind with Selena -- even

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