The 55-year-old woman, who lives in Germany, is suspected of being involved in his death.
Her daughter, the wife of Van Seggeren, had already been arrested in December. She had to come to court on Wednesday in Leeuwarden. When her mother wanted to attend the session, she was also arrested. The Public Prosecutor does not want to say exactly what the mother-in-law's involvement would have been.
Van Seggeren (37) was found dead in July in a pasture in the village De Westereen. He had attended a music festival the night before. He would be picked up, but nothing was heard from him after leaving the festival.