A LETTER to Marilyn Monroe from one of Robert Kennedy's sisters, which appears to support the stories of a relationship between the actress and the attorney-general, was to have been auctioned in Hollywood yesterday, but the sale was blocked by a Los Angeles judge after Monroe's estate filed suit. Jean Kennedy Smith's 1961 letter, on the family's Palm Beach stationery, reads: 'Dear Marilyn . . . understand that you and Bobby are the hot item] We all think he should bring you with him when he comes back East . . .'