I split up the family??

I posted yesterday (Would you be upset?) About my mother allowing my sisters abusive ex husband around our son while she was watching him.

My mom text me this morning, finally realizing that oh yeah our son was at her house (which we already knew) but that he was upstairs the whole time so it makes it ok because the guy we dont want him around was downstairs?? Oh and she thanked me for splitting up the family.

Yes it's my husband and I's fault for being mad that my mom allowed this guy in her house around our child without telling us. We split up the family because we actually said something about it.

I just don't get people. I don't understand how some people can't ever see fault in there own actions. And how I'm just suppose to be ok and not care because he was there for only 5 seconds??