What is the ruling on siblings ignoring their mother's questions, and should I intervene to advise them or leave them be?

Your brothers' avoidance of answering or asking their mother is blameworthy, and it may be considered a forbidden act of disobedience if the mother is harmed by it. They must fear Allah and be good to their mother and show her kindness. You should advise them by explaining the seriousness of disobeying parents and the virtue of honoring them, as it follows the command to worship Allah alone in the Quran, as Allah Almighty says: "And worship Allah and do not associate anything with Him, and be good to parents."