Complex sentence: Avis Graydon died in 1973, without ever knowing that her sister had been reburied in Brookwood, thus she never had the opportunity either to visit the grave or to erect some form of memorial over it.
Split into simpler sentences: Avis Graydon died in 1973, without ever knowing that her sister had been reburied in Brookwood. Consequently, she never had the opportunity either to visit the grave or to erect some form of memorial over it.