Rory and Sean will thus become the 19th set of brothers to have played together in the same Scotland team.
Scotland coach Matt Williams denied he was taking a chance giving Rory his debut against a confident Welsh side, overwhelming favourites to carry on their unbeaten run in this season's Six Nations Championship.
He said: "If you're good enough, you're old enough.
"Since I've been here I've given around 12 players their opportunity and 12 months ago nobody had heard of Chris Cusiter and Allister Hogg, but look where they are now.
"We wouldn't be giving Rory the opportunity if we didn't have the evidence that he could handle it.
"We've been lucky for players to break the line and be aggressive with a good kicking game and strong running.
"And obviously we haven't been happy with all the performances recently.
"Rory is a young player we've been watching this season and we are impressed with what we've seen.
"We've had him in the squad from the beginning of the Six Nations tournament and we just felt it was time for him to step up and take his opportunity.
"We're fortunate to have someone like Rory, and his brother Sean, with their physique, power and pace.
"The roll of winger and fullback has changed dramatically in the last five or six years and our opponents, Wales in particular, all have a strong kicking game.
"So you need powerful and skilful players and Rory will have his chance to show what he can do."
Scotland
15 Chris Paterson (Edinburgh Rugby)
14 Rory Lamont (Glasgow Warriors)
13 Andy Craig (Glasgow Warriors)
12 Hugo Southwell (Edinburgh Rugby)
11 Sean Lamont (Glasgow Warriors)
10 Dan Parks (Glasgow Warriors)
9 Chris Cusiter (The Borders)
1 Tom Smith (Northampton Saints)
2 Gordon Bulloch (Glasgow Warriors) CAPTAIN,
3 Gavin Kerr (Leeds Tykes)
4 Stuart Grimes (Newcastle Falcons)
5 Scott Murray (Edinburgh Rugby)
6 Simon Taylor (Edinburgh Rugby)
8 Allister Hogg (Edinburgh Rugby)
7 Jon Petrie (Glasgow Warriors)
Substitutes
16 Robbie Russell (London Irish)
17 Bruce Douglas (The Borders)
18 Nathan Hines (Edinburgh Rugby)
19 Jon Dunbar (Leeds Tykes)
20 Mike Blair (Edinburgh Rugby)
21 Gordon Ross (Leeds Tykes)
22 Andrew Henderson (Glasgow Warriors)