Me time for mom? Is it possible?

I just walked in the door ten minutes ago from my last cl of the day. I am expecting twins (unplanned miracles) so I have loaded myself up with cles right now, so that I can graduate university earlier. My fianc� and I have full custody of my 11 year old sister. My fianc� is an electrician and works from 7am to 3pm, when he picks up my sister from school, and comes home, where he proceeds to unwind. My sis watches TV and then gets to work on her homework. I am at uni from 7:30am to 6pm. When I walk in the door the first thing I hear is 'what's for dinner?' and 'can you unload the dish washer, there aren't any clean cups'. Why is it that my hubby and my 11 year old understand that they need to unwind, but don't seem to think that I do. When I come home from a long day, I just want ten minutes to myself. Is that so much to ask? How can I help my family understand this?