Mother in law problems!!

Yesenia • Engaged to the love of my life! Full time student and mother to an angel baby that will always hold a place in my heart, a beautiful 9 yr. Old daughter, and an awesome 11 yr. Old stepson!
Ok so my mother in law lives with me and my fiance and doesn't help with any bills whatsoever and it's gotten to the point that I can not stand her living there ever since things turned bitter between us. I've asked my fiance to please ask her to get her own place but he is reluctant to tell her. So I was finally able to convince him and he gave me the ok to find her a place for rent and got my hopes up! Well now it turns out he wants to buy her a house but wants me to help pay for it when I'm going to school and only working part time. I feel like its just so unfair and I don't know what to do anymore... :'( What would you do in my situation?? Thank you in advance for your advice.