Asking a family member to pay rent...?

Four months ago my stepdaughter, her boyfriend and their (then) two month old baby moved in with my husband and I to "get their finances in order." They pay no rent or board of any kind. I am a teacher, my husband does consulting work, but we live very paycheck to paycheck. What do you think about charging them room and board? They both earn about 3,000 a month together and have a 700. mo. car payment. I am upset and angry about this arrangement. I feel they are taking advantage of us.