Reading your post it sounds like your sister felt pretty bad about it and accidents happen.
Sometimes my dog gets into things and I don't notice for a while because I'm not constantly after him every second of the day.
You're also very good to let your sister come over and eat rather than go without. She doesn't sound lazy. She was on her way to work and that's why she couldn't help you pick it up when you told her about it.
I hope you were able to get everything cleaned up and nothing was lost to this accident.
And maybe in the future your sister could go through the house and close all doors so the dog doesn't get into anything else. :-)
@Nickey - She didn't do it on purpose. Taking her key is a little harsh.
Next time just tell her if she is going to bring the dog, which you wish she wouldn't, that he must stay in the kitchen with her or not come at all.
Fiancé & I live with my MIL & she has 5 dogs. All of them piss in the house. Every time if let them come down stairs they would piss every where! When I found out I was pregnant I said no, they cannot come down here any more. I don't need to be cleaning up after her dogs.
Yes, accidents happen and I understand she wasn't to blame for the dogs actions. She's just so irresponsible and that's disappointing. I told her she's welcome, but no more dogs in the house. It's a start, right? She's lucky my hormones are in check this week, ha.