He said he will talk to her about it as well.

You are right, I would see it as a red flag myself but he did handle things when she last disturbed his life. She spread rumours that he was pursuing me when I saw him as my sister’s friend and nothing more. His flatmates started seeing him as clingy and make fun of him. He sat them down and spoke to them about everything. His flatmates stopped gossiping with my sister about this relationship at least. My sister was a close friend so naturally she knew his secrets. She bared them all to his friends and flatmates. Everyone has a past mixed with some proud moments and some not so proud moments. It feels frustrating when we cannot control whom we share them with and whom we don’t.

He also helped me deal with my side when it was too much since my sister had taken to a point where it was too much. She had gone through my old phone when I was in college, two three years ago. I had no idea about it. She saw my conversations with my ex and shared them with my mother and when she saw even that didn’t bother me, then with my dad as well.