Maumalo made his NRL debut in round 10 last year, making a total of eight appearances for the first grade side. He impressed in his early outings, only to struggle later during the team's long losing streak.
Fellow youngster Sipley made some eye catching charges in the round nine win over the Dragons and, like Maumalo, has undoubted potential.
"They're massive units and Ken and [Toafofoa] certainly bring lots of enthusiasm, not to mention size," said assistant coach Justin Morgan.
"Ken's ball carrying is his strength and he'll help us get out of trouble with his runs from our end. His defence has also been improving and he has worked hard to earn this opportunity. [Toafofoa] had an impressive debut against the Dragons [and] has been pushing for another chance, too."
Ben Matulino comes into the starting XIII in place of State of Origin bound Jacob Lillyman, with Sipley joining an otherwise unchanged interchange bench.
Warriors v Sydney Roosters
Mount Smart Stadium, Auckland
6.10pm, Sunday, June 19
Warriors:
1. Tuimoala Lolohea
2. David Fusitu'a
3. Blake Ayshford
4. Solomone Kata
5. Ken Maumalo
6. Thomas Leuluai
7. Shaun Johnson
8. Albert Vete
9. Issac Luke
10. Ben Matulino
11. Bodene Thompson
12. Ryan Hoffman (C)
13. Simon Mannering
Interchange:
14. Nathaniel Roache
15. Sam Lisone
16. Toafofoa Sipley
17. Charlie Gubb