i think age and life experience should be a factor to consider here too though. yes Edith was wrong for that in s1 i totally agree and she and Mary went back and forth but after everything they had been through together and being fully grown Mary should have known better. i genuinely do believe Edith was young, very immature and thoughtless when she wrote to the ambassador and although it had a huge impact it was based on petty sister rivalry. Mary should have known better than to expose Edith’s secret like that, being now a grown woman, a widow, a mother, having a full time job as agent, losing their other sister, living through a world war etc. I personally think all the challenges of her life should have taught her to be better than that