What should I do regarding my daughter's visits to her father and paternal grandfather, especially since her grandfather teaches her wrong principles and negatively affects her psyche and behavior? Should I prevent her from visiting and sever ties of kinship, or should I let her go and be exposed to these ideas?

There is no doubt that what was mentioned about your daughter's paternal grandfather — his abandoning prayer, drinking alcohol, and teaching things that contradict Islamic morals — are matters that do not inspire confidence. Raising children is a great responsibility entrusted to the parents. If they separate, the mother has a greater right to the custody of her daughter, unless she remarries or has a legitimate impediment. This does not mean cutting off the relationship between the daughter and her family. However, if the father or grandfather is not upright, then the connection should be limited to what is obligatory, and the daughter should not be left with them for a period that would allow her to learn from them what contradicts Islamic morals.