If you’ve read His & Hers by Alice Feeney can you (without spoilers) tell me if Rock Paper Scissors has a similar ending?

I picked up Rock Paper Scissors without realizing it’s the same author as His & Hers and I don’t want to waste my time reading it if the ending is also kind of unbelievable. The twist at the end in His & Hers was that the grandmother with dementia was the killer. Really? I felt lied to. She was portrayed as someone who couldn’t even really walk around her own house. I like twists to at least be plausible.

Don’t spoil it please, but is the ending believable?