Having formerly had a great interest in the more bizarre forms of stage magic (i.e. apparent psychic readings, mind control and stage hypnosis), I can guarantee you, that there are thousands of seemingly supernatural and impossible feats that can be accomplished, using nothing but psychology, misdirection and a good deal of confidence.
While I have not yet come to any conclusion about whether demons have the power to effect such things by darker means, such would surely be the exception rather than the rule. I think you can safely assume trickery in most cases. “Cold reading” is another bag of monkeys entirely, and is used (consciously or not) by different types of readers. You can get stunning results without sacrificing any goats.
However.
Opening yourself to spiritual influence is a very dangerous thing, and while the medium is most likely a charlatan, the “forces” that they open themselves to, are very real. Also, the charlatan might be a thoroughly unpleasant and sinister person, who manipulates your family to gain power and money (my impression is, that astrologers, tarot readers, aura readers and the like are quite often well intending but misguided persons, while mediums and faith healers are generally charlatans… there is of course a whole spectrum in between as well)
I recommend praying for your family, for the protection of St. Michael.
