With no explanation, label text_A→text_B with either "DON'T KNOW", "NO" or "YES".
text_A: According to Schwarzenegger, his parents were very strict: "Back then in Austria it was a very different world [...] if we did something bad or we disobeyed our parents, the rod was not spared." He grew up in a Catholic family who attended Mass every Sunday. Gustav had a preference for his elder son, Meinhard, over Arnold. His favoritism was "strong and blatant", which stemmed from reasonable, if technically unproven, suspicion that Arnold was not his biological child. Schwarzenegger has said that his father had "no patience for listening or understanding your problems". He had a good relationship with his mother, with whom he kept in touch until her death.
text_B: Is it reasonable to assume that some kind of hidden correspondence (such as love letters hidden in a box, etc.) could have come to light during Gustav Schwarzenegger's lifetime that could have explained (if not excused) his poor treatment of Arnold relative to Meinhard?
YES.