You were a lot more graceful and calm than most people would have been. Your SIL is clearly trying to manipulate you by cheating on you and then using her feelings against you when you stop paying attention to her. She doesn’t feel bad about what she did; she’s just upset that she can’t control how other people see her.
Setting clear limits and keeping your peace was the right thing to do. It’s very bad that she tries to make you feel bad or tell your brother to stop talking to you. It sounds like your brother is stuck in a cycle of emotional control and being alone, which is something abusers do to stay in charge. Keep helping him from a safe distance, check in when you can, and let him know you’re always there for him without being pushy. You aren’t the issue here. You won’t let your brother get used to betrayal and manipulation, which is exactly what he needs as a stable, grounded presence.
UPDATE – AITA for not talking to my SIL for cheating on my brother?
