What is the reason for the mother's strange behavior and sudden isolation that began after her grandmother's death and reappeared after returning from travel, and what is the optimal way to deal with this situation?

We advise you to take your mother to a psychiatrist while also performing Ruqyah (Islamic exorcism) on her with legitimate incantations. Advise her to maintain the obligatory prayers, as they are the key to all good and the best aid in hardships and calamities. Allah Almighty says: "And seek help through patience and prayer. And indeed, it is difficult except for the humble [in prayer]."