According to wikipedia
In European folklore and folk-belief of the Medieval and Early Modern periods, familiar spirits (sometimes referred to simply as "familiars" or "animal guides") were believed to be supernatural entities that would assist witches and cunning folk in their practice of magic. According to the records of the time, they would appear in numerous guises, often as an animal, but also at times as a human or humanoid figure, and were described as "clearly defined, three-dimensional? forms, vivid with colour and animated with movement and sound" by those alleging to have come into contact with them, unlike later descriptions of ghosts with their "smoky, undefined form [s] ". Source: wikipedia
I don't think they can turn on you since they are assisting you with your magic, however, I do know you can't contact them, but you might be able to in the Astral Plane, but also know that familiars may not speak english, since they are usually animals.
I hope this helped and made sense.