Agree with all those who say to work out ground rules beforehand, including whether or not you'll be doing any babysitting and how much, etc. She might want you to watch gs every evening, for example, while you assume you'll only be called upon once or twice a week. Or you may expect to get to babysit and she intends to live a very separate life from you up there in the attic rooms.
Imo, deciding on whether or not she'll contribute financially is also important, as is whether or not she'll be responsible for any household chores. You 2 might have to make some compromises on this. But better to get it all out on the table beforehand.
If you can't work everything out, then, that's your cue that it just can't be. Also, be prepared for some things to change after she has been living there a while.
I also agree with the idea of an end-date, at least as a goal. In fact, maybe you should have an even shorter "trial period." If things are going ok by the end of that period, then you continue. If not, she and gs would have to move out.
Just some thoughts... But first, imo, you need to discuss this with your partner. What does he say? And if he's against it, can you find it in your heart to turn dd down? Or does she come first with you?