You did the right thing by staying calm and calling her on that. If she gets pissed over something like this so be it. You remain calm and strong.
You were not snooping, your son simply stated his concern and you did the right thing. On an emotional level she is a teen. You will see this side of her. It's important not to throw the teenage behavior in her face, it won't do any good, but try and understand that is where she is right now. Clearly not rational and wrapped up in herself.
Now that you called her on that and she apologized to your son, try your best not to bring it up. It's selfish, foolish, and unfair but bringing it up again will make things worse.
Continue to take care of those kids and don't say anything negative to them about her. They need you, be their rock.
