No, it is not permissible for any one to cause distance or cut relations between relatives. It is a major sin which is mentioned in Quran with a curse from Allah on the person cuts relations with relatives.( Sura 47, verse 22 and 23). Even if her father in law is oppressive, he remains related to her child and depriving him from seeing his grand son is a major sin. The Prophet Muhammad (SAWA) did not stop his wife Um Habeebah from seeing her father Abu Sofyan who was one of worst enemies of the Prophet. He allowed her to receive her father in the house of the Prophet.