Please tell your mother to Pavana with Na Mo Bud Dha Ya and ask for THE POWER OF LORD FATHER BUDDHA sending all her love and good wishes to her both babies to Motana boon and the 2 babies should be gone to the heaven (happy place.) after recieving Boon Pavaan from your Mother.
2. It was the accidental death for the third baby, so ypur mother was not wrong.
3. You have nothing wrong but you should Pavana and send your good wishes with Na mo Bud dha Ya and ask for to Palung Pra help your Mother and the other 2 babies to find the right way to Nibbana.