Is a frater-soror marriage permissible for Catholics?

By "frater-soror" marriage, I assume you mean "brother-sister" marriage, a marriage in which the couple does not engage in marital relations. Brother-sister marriages (also called Josephite marriages in honor of the marriage shared by St. Joseph and the Blessed Virgin) are rare in the Church and should not be entered into without spiritual direction from an orthodox, balanced spiritual director. They are usually entered into when one or the other (or both) of the partners is highly-advanced in the spiritual life and it is determined that a brother-sister marriage would advance the spouses in the spiritual life.