What is the ruling on taking pressed olives from my mother if my parents only pick their share of the olives jointly owned with my uncles, who have permitted the picking, but without their knowledge?

If the questioner's parents take their share of the olives, or take what they have been permitted to take, then there is no harm in dealing with them in terms of gifts, presents, and the like. The basic principle is to accept and believe the word of the trustworthy.