。
My uncle is coming to visit us tomorrow.
Literal
Tomorrow uncle [subject-が] us [object-を] visit come [nominalizer-こと] [dative-に] become-is.
訪ねて来る chains 訪ねる ('visit') with 来る ('come') — 'come visiting.' The ~てくる auxiliary here signals motion toward the speaker's location. Then ~ことになっている again for the established arrangement. Note 'uncle' is the subject (marked with が), while 'us' is the implied destination of the visit.