Complex sentence: They recognise him as the son of Yang Kang and Mu Nianci, after seeing some resemblance to his late parents and demonstrated some combat skills only his mother would knew, and take him into their care.
Split into simpler sentences: They recognise him as the son of Yang Kang and Mu Nianci, after noting that he resembles to his late parents. Their suspicions are confirmed when Yang demonstrates some combat skills that only his mother knew, and the couple take him into their care.