How sensible of you to post this and to seriously think this through rather than be steam-rolled into something you're not entirely comfortable with. You say D lived at home 7 years ago and was spiky and difficult so I understand your reluctance. Have you considered looking at alternatives.
Gabriella G posted good advice about getting rid of the damp in D"s flat.
Or perhaps you could help D find another flat near to you.
And/Or help her out financially.
IF you decide to have D and GS live with you ....and because the "2 years" isn't an exact time, I would put it on a legal footing by having a 6-month tenancy agreement and charge her rent to cover her share of utility bills. If you all find things work out you the agreement can be renewed or not if D is spiky and difficult. Someone in an earlier post suggested D keeps her flat on for a few months. A good idea I thought.
Another sensible post said "listen to your inner voice".
Lots of great food for thought from Gransnetters .
I hope all goes well.
They don't really care do they
Things you find stressful that other people don't notice.