My late aunt was in that position with her Spanish fiance back in the fifties. She ended the engagement rather than be subject to his parents wishes all her life. She never married, either officially or otherwise, but she had a successful career as a university lecturer. She was a good aunt to her nieces — in my case, more psychologically supportive than my mother when things got tough.
She also never explained to her family why she had broken off the engagement, but when she was in her sixties, she told me the thought of the would be mother-in-law and all his sisters having expectations of her that she didn't have of herself, or want, was what did it. Strong woman.
Parents don't own their children, even when they think they do.
I wish your son and daughter-'out-law' all the best, joan, and hope they do what they want. She is not living in a Chinese culture now, presumably, but an Australian one. Her parents need to accept this. Presumably the young couple could have a quiet registry office wedding — if married is what they want to be — without the hassle?