They may be a bit on the tart side and might cause sores to lip and mouth; I would be careful in not giving too much too often.
However, branches, bark and leaves are fine, so whenever your father-in-law is cutting the trees or collecting dead wood, ask him for it. You can try some dead leaves as well - when I had roaming piggies in my garden, they loved munching on dry leaves!
PS: Unsprayed roses are also fine to give, broken blooms, stems and leaves fresh, or dry; but make sure that you have removed any thorns.