Please don't feel guilty for needing a little downtime from the little ones. We all need it and get it when we can. Especially when we are the primary caretaker, we need time to take care of ourselves to recharge our batteries and re center ourselves. Enjoy it and don't think twice about it. You deserve it.
I am sorry things are still crazy in Casa Brubeck. Your W sounds like she needs help.
Like 25 says, the way she is acting toward her children is unnatural. Has she always been like this?
Congrats on the 2nd job interview! I will send up a prayer that you get it.
It sounds like you have a secret ally in your MIL. And the way you describe how your SIL popped in and then popped out it does sound like your MIL might have had her check on W while she was gone. Your MIL knows what is up and she probably doesn't just see it with the way your W is acting; she probably sees how your children are reacting to the situation. It has been that way in my sitch. H's family has seen how S behaves around H and they know.