Lord of the Rings, Star Wars, and Harry Potter at least one very useful thing in common. A mentor/guide for the journey (Gandalf, Obi-wan, Dumbledore)
This will give you a means to keep them on track, should they become stalled or distracted. Like the mentors mentioned, the mentor in your story should have a reason why he can't bust out in awesomeness and win the day, thus it falls on the heroes. DM self agrandizement, even just percieved, is terrible for the morale of a group.
The mentor/guide could be the head of a guild order, and he is observing a group of initiates (players) to see if they are ready, or could be old sage who is the only one who knows how to open the
Vault of Annarath but hasn't much strength and needs a bunch of strong lads to get him there and then help him haul out his treasure. Whatever you choose, it should be someone with implicit authority of some sort over the party, even if he never really wields such authority.
Also, let the roles guide you.
- Controller - Has the least simple task, and player must accept that damage is not the character's primary function. If you use this one, give it to the most experienced player. But I think that in a small group of newbies, the controller is the least necessary. Maybe make your mentor a NPC controller so he can hop in and save the day if the adventurers are at risk of loosing. A controller NPC is less likely to steal the glory from the players.
- Defender - Feels like it should be doing damage, but that is not really what the defender is about. A good defender is nearly as complex as a controller, less optional in a group, and easy to initially mistake for a striker (and that usually ends up being disappointing) Make it clear, if you use this one, that this guy is the one keeping the others alive. This would be a good one to give to the experienced player.
- Leader - So long as it is stated up front to all that play that the leader indirectly wins the fight for the party by enhancing others, then this can be a fun role for certain personalities. The classic leader will of course be the cleric, but you could really call any of them whatever you want. This would be a good one to give your sister, as it allows for both nurturing as well as directing. Nearly all mothers I have known would fall into that role quite naturally.
- Striker - The most direct role by far. You are there to hurt things until they don't hurt you any more. Strikers get the big damage numbers, and the most glory. Honestly, strikers are fun. You don't need to know much to pull off a striker well. This seems like it would be the best option for the skeptical brother. Since he is into athletics, I would stick with either a Ranger, Barbarian, or Rogue. If he is into football or wrestling, Barbarian. If he is into baseball or basketball, Ranger. If he is into soccer or tennis, Rogue.
Also,
do not let them die! Fudge the dice, grant action points for cool stunts, intervene with an NPC, whatever. You, as the DM, are an entertainer and are allowed to do whatever it takes to make sure they are having fun. Your job is to make them feel like they have an overcome difficult situations because they are awesome, both as characters and players.
Use game aids liberally. I suggest power cards, because nothing kills momentum like having to search through the books, and like a party, you don't want this to stall out.
Finally, I would suggest starting at 3rd or 5th level. That way the players are not out of options by the end of the second round.