I would actually recommend Burdock (Arctium lappa) root in tincture, tea/infusion, or cooked root aka "gobo" form.
Personally, I would drink the infusion (made by steeping 1/2 ounce dried root or 1 ounce fresh root in one quart of freshly boiled water for four hours or overnight), two cups a day. Even after the psoriasis goes away, this is a good general liver supporting infusion.
If you can only find the tincture, 20-30 drops twice a day should be sufficient.
If you can find the fresh root (it's hard to find around here, and not really in season), eat as much of it as you want at least three times a week.
Good luck to your SIL!
