Regarding irregularities in care of my granny by her son

My granny,a highly aged lady(90+ age), who is also suffering from Parkinson's Disease since over 5 years now, she is in a deteriorated health condition and is being taken care of by three of her sons out of four, and the fourth son has never come forward to undertake his responsibilities towards her, neither economically nor physically in providing for her maintenance and care. Two of the sons have resided in the House inherited from their father, which is also the place where my granny is presently living. She has until now been taken care by the three sons with a mutual understanding between the three. The fourth brother has never stepped up, despite been repeatedly called for assistance. Please direct me on the rights my granny and her three sons can exercise in this case, with due consideration to my granny's ill condition which debars her from making a move herself. Moreover, could a lawsuit be filed by someone on behalf of my granny, and if so, who could possibly exercise this right?