The pair both attended Whitchurch High School in Cardiff, a school that also includes the likes of Olympic gold medallist, cyclist Geraint Thomas and Welsh rugby league international Elliot Kear as former pupils.
Sam's good news followed just two days after Gareth was named PFA Player and Young Player of the Year.
"It’s fantastic news for Sam," said Gareth. "It’s probably the ultimate accolade in rugby to be Lions captain and he deserves it.
"He’s done fantastically well for Wales, captained them, won the Grand Slam, the Six Nations and he’s still only young.
"This is another massive achievement and I’m sure he will take it all in his stride and end up being the best player out there.
"Sam was always good at his school work, which I wasn’t! He was great in every sport.
"He was an all-round sportsman and he’s showed that in his rugby. He’s very quick compared to many players in his position. He’s doing fantastically well."