I don’t think you should’ve applied this pressure on her when she didn’t add your sister.
I think she sensed that you aren’t treating her as an individual with her own judgement on who she cares to be friends with and just assuming she owes you becshse your brother did her the favour of marrying her .
Did your sister make much of an effort with her outside of Facebook before marriage?
I tried to make effort with my SIL when my brother was marrying her, she snubbed me and gave me a speech about how I’m coming in between her and my brother (merely for asking a simple question while getting to know her for the first time).
When I had a child she decided that she wants to give me friendly personal advice about my fertility journey and children.
I fell for it and decided to humour her conversations.
When she had her first born, I was excited to know more about my neice and wanted to contact my brother who was sharing information about his daughter with me, simple ones..
She made a fuss about how it’s not my place.
So I decided she isn’t material for friendship and now I ignore her faux friendly attempts
Focus on ur brother OP. She sounds insecure