Newlyweds.. and living with my mother in law?

My husband and I live in my mother in laws home for the purpose of saving $ for a downpayment on a home that we are in the process of getting pre-approved. Well in the house there is alot of people living there. My MIL and FIL my Sister in law her two kids who are 18 and 20 that don't work and my husband. Well we recently moved in a month ago and since my MIL is not giving mortgage payments and is going through the process of mortgage modification program to pay less rent i figured she would charge us a lil bit of rent due to fact that my husband and I live in one room and knowing that we are trying to save. Well today she said that we are going to pay $500 plus light bill/mth. I know this may not be alot to some people but when you have other people living there and are not paying the same amount of $ or being equal feels like its not fair. My MIL has helped her son at times through his young adulthood and now she wants him to recompensate the $ she has giving him. On the other hand, my parents helped me out in my ypung teens and early 20's by not charging me rent or lets say i didnt have $ for something they would treat me. But never have they asked me for a penny back. My parents are not rich it's just how my mom said that any mom helps their kids move forward in live by giving them that little push. My sis in law works a lil bit of hours and believes that my husband should pay more rent cus he wins more. I just see it as being equal. For those who have lived with their MIL, what do you think about this situation???