Cheating sister in law has affected me.

Cat
So basically my sister in law cheated on my brother. Idk why but I feel particularly bothered. They have 3 kids together all under 7 years old. I guess it's just disturbing that someone could be so irresponsible and immature to do something like that. My heart breaks for those kids. 
Why does this bother me so much though? I was not close to that sis in law. I'm not suspicious of my boyfriend cheating on me though I will admit I'm insecure. I've had nightmares about him cheating on me prior to this. I'm afraid I'll definitely have more now. Idk how to explain what I feel. Just bothered and anxious. I want the feeling to go away.