The fatal collision occurred on 28 December, with Cuney dying on 30 December and Pierre succumbing to her injuries on 1 January.
A man from Haddenham was arrested on suspicion of causing death by dangerous driving, possession of class A drugs, and drug driving; he has been released on bail.
Cuney, a teacher who relocated from America, reached the 2024 Britain's Got Talent final as part of the choir NSO (Northants Sings Out).
Both families have paid tribute, remembering Cuney as an "energetic, powerful presence" and Pierre as "completely selfless" and the "centre of our family".