Your relative's stealing may also have an addictive quality to it. This might best be approached through counselling and possible involvement in a twelve-step program. Stealing becomes an experience of power, albeit unhealthy power. The reasons behind the sense of powerlessness which may give rise to the stealing need to be explored through counselling, possibly to include family counselling. Your insight regarding a possible spiritual dynamic is on target.
Addictions have a strong spiritual component.
Addicts of any sort are looking outside themselves for something to comfort the, make them feel better. To break the back of the addiction, a spiritual solution is often indicated. In the same sense, the solution to fear and powerlessness is often a spiritual one. Healthy power is based among other things on a good sense of competence and giftedness. Finally and obviously, if your relative is open to it, prayer, especially healing prayer may at least be a comfort if not an open door.