Step Daughter and school work

My fiancé's daughter is 8. He has full custody of his daughter and allows her to do a week at her moms and a week here with us, all year long. She goes to a private school. Which is just around 10,000 a year. 
Well when she is here with us, I read the school letters, and I stay on top of her to get her work done and to study. However when she goes to her moms, she doesn't do anything at all. Her work never gets completed or turned in. Her spelling tests come back with maybe one right, where as on our weeks she is getting 100s on them. She has a monthly book log, where she had to read a book a week and take a test on two of them. Well they don't get read at her moms house, causing her to have to read double when she is here. She had a science project earlier this month, that I worked on with her completely. It is just getting ridiculous. Her mother doesn't work. Yes she is 8 months pregnant but so am I. I just don't know what to do anymore. My fiancé has talked to her mom numerous times about this, and though she acts upset and like she will talk to her daughter and appropriate punishment will be set into place, nothing happens. So of course I have to be the bad guy. Well now it is her spring break this week and it's our week, and I'm going to have to make her do the work she missed, plus dish out punishment for not doing what she is suppose to do. Smh just getting really frustrating!!