What is the Shar'i ruling on the questioner and her sisters refraining from visiting their mother in her home, where their drug-addicted brother, who molests children, resides, due to their fear for their children, and is this considered an act of disobedience?

Parents are responsible for their children and must protect them from anything that could corrupt them. If keeping the children away from their maternal uncle serves their best interest because he is a source of suspicion, then do not hesitate to do so. The grandmother's opinion and anger are of no consequence, as you are the ones responsible for the children. You can visit the grandmother when the uncle is not present, or without bringing the children, or the grandmother can visit you at one of your homes. If a visit is necessary, you must not allow the uncle to be alone with the children, even if the grandmother gets angry and supplicates against you; her supplication will not be answered because it is a supplication for sin.