P&B shakes do not cause this. The original Metamucil was psyllium with other things added, but they have several newer versions that are not all natural. Same with Citricil.
Also, if your sister-in-law is having trouble with constipation, high-fiber intake will only make it worse. Fiber is great for keeping your regular, but it's best to get things moving and clear out any impactions before beginning any fiber therapy. See my suggestions in my other post for more information about this.