brother had Mom with dementia sign quitclaim for half her house with survivor benefits. Will says 4 way split. Claims escrow instruction will resolve

Three years ago younger brother had our 90 year old Mom with dementia sign a quitclaim deed for half of her $600k house with survivor benefits upon her death. None of the three remaining siblings were aware of this action until August of 2019. He claims he didn't know there was a will and was trying to avoid probate. Her will states equal split among 4 remaining children. He claims that the property will be equally distributed per his escrow instructions upon her death and that this can be done only if the will is uncontested. He's a 20 year IRS guy. I think he's lying. Am I wrong? Older brother (Mailman) is the executor of the will, is completely inept and believing every word.