How to convince fiancee to have his mother move out

I am engaged to a great guy - the problem is he moved his mother in 4 years ago. She acts like she is an invalid, but she is not (I have caught her sneaking upstairs to our bedroom when she thought no one was home - and that is just the tip of the iceberg with her). She acts this way so he feels like he must take care of her. I feel that to build our lives together, we need the house to ourself. She is ALWAYS around - we cant watch t.v. together with out her. She never leaves the house, and the minute he comes home from work she clings to him, as if she is the woman he is coming home to. I brought up to her that she needs activities outside the house so that her son and I could have some alone time, and she burst into tears saying she can't afford a nursing home. While I WILL NOT fall for these guilt antics, my fiancee does. I ended up postponing our engagement and moving out over his inability to even ask her to let us have an evening alone. He is now saying he has talked to his sister, and the sister will take in the mother so we can build our lives together. The problem now is, he constantly makes excuses on why he can't bring this up to mom. He keeps procrastinating the discussion with her - and saying he doesn't want her to "feel bad" (as if I am asking him to yell at her) He understands and agrees with me about having the house to just ourselves, but he also doesnt understand why the 3 of us can't be happy together under one roof. He keeps telling me he loves me, and wants to marry me, and asks me to come over every night, but until I know it will just be him and I having a life together, not the 3 of us, I can't marry him or move back in. He has broken down in tears begging me to come back - How do I make him realize that he must do something soon or else he will lose me?

Ask him to ask his sister to call the mother there under some pretext to stay with her. Or if required and possible you call up and speak to the sister and explain it all. However do it delicately and do not pour all the frustrations into it.