Gwent Police say the 15-year-old girl from Newbridge was arrested on suspicion of threats to kill, possession of a bladed article on a school premises and conspiracy to commit murder.
The 14-year-old girl from Risca was arrested on suspicion of conspiracy to commit murder.
More top news
Brooke, from Merthyr Tydfil, was last seen in the early hours of Saturday 12 October.
A dry night for many with clear spells and possible fog patches developing by dawn.
Aaron Ramsey has been ruled out of Wales' Euro 2020 qualifier after he suffered a thigh injury last weekend.