Hi Mai,
I definitely would never say that it is OK for you to work more than 10 hours per day. I know that you feel that you were doing them a favor by taking care of the dog while they were away and Mary Jo did not acknowledge that favor. Instead she made you feel like that wasn't a big deal. I would suggest that you don't get mad and instead sit down with her and tell her how she made you feel. Explain that this is not part of your job and you didn't mind taking care of the dog, but you felt like she didn't appreciate that you did that.
As for working over 10 hours per day...I think that what she is trying to say is that she might need a little more time from you over the next few weeks until the kids go back to school. But after that, maybe her schedule becomes lighter with you not working as much. This happens with some au pairs who are OK with it because for most of the year the schedule is less hours. It is not something that we say is OK but many times the au pairs and families agree to this arrangement. Mary Jo probably felt like she was giving you extra time off over the weekend that they were away. If she wanted to, she could have had you do all kinds of projects like straighten the kids toys or straighten their closets or drawers or laundry for the kids while they were gone. The dog is a responsibility too so I understand what you mean when you are saying that you did work over the weekend. But she felt like she was being generous in other ways too.
I know that you just got here so I would encourage you to stay calm and see how the schedule changes in a few weeks when the kids go back to school. The relationships that work best are the ones where au pairs and host families help each other....the host family should be generous with the au pairs but the au pairs need to do the same. If both au pairs and host families keep score all the time and don't want to be flexible or generous with each other, then the relationship will fall apart. I don't want anyone to feel like they are being taken advantage of either so there has to be a balance. Mary Jo should not have said the things that she said to make you feel unappreciated but I think that she is trying to say that she needs you to be flexible now and that it will be different for you soon when the kids are back to school. I hope that you can work this out with her. I will be coming for a Two week visit on Monday so we can talk more then or feel free to call me.