Brother and sister duo Spencer and Maddie were seen last month in the first official promo for the 2013 season, turning up on the doorstep of newlyweds Roo Stewart and Harvey Ryan.
In an interview with Digital Spy this week, McGranger - who plays Irene Roberts - promised "intrigue" from the new arrivals.
McGranger revealed: "They arrive with a secret. That's all I can tell you - that there's a secret that is revealed within a couple of weeks. They're both terrific actors - terrific little actors.
"Andrew [who plays Spencer] is playing much younger than what he is - I think he's about 23 and he's playing 17. Kassandra is playing Maddie, and I think they'll be a terrific addition to the Home and Away family with the intrigue and the secret they have."
She added: "Much later on down the track, their brother Chris turns up and there's a bit of drama there, too."
The arrival of Spencer and Maddie comes after show bosses promised to feature more fostering storylines and increase the number of scenes at Summer Bay High.
Asked whether Home and Away's writers are trying to take the show back to its roots, McGranger replied: "Oh, absolutely. I think there's a few things that are important to the success of a show like Home and Away. I think the viewers like the stalwarts, they like the lynchpins - the Alfs, the Irenes, the Leahs… and of course, the Marilyns, the Roos and the Harveys.
"Those characters are important, but the young ones are the ones who, if you like, life happens to! They circle around the lynchpins, and I think the more young people you have on the show, the more opportunity there is for that kind of drama."
Home and Away airs weekdays at 1.15pm and 6pm on Channel 5 in the UK. The show returns for its 2013 season on Network Seven in Australia in late January. Get the Inside Soap magazine on your iPhone or iPad