B.K.
I know this isn't going to be much help for a single mom who dates much or brings men home, but since I am assuming you don't (hoping you don't ... no offense), I will give this suggestion.
If your son is in a toddler bed, or otherwise small bed that might fit in your room, why not move him in with you, and then your daughter won't have to move. This seems to me to be the easiest suggestion, since your son comes to bed with you in the night anyway. Why not just put him in there? Then he won't be roaming the house to come to you, and your daughter won't be woken up by him. Best of all, you won't have so much change to contend with, and you won't have so much worry keeping you up. Since your sister is only going to live there for a while, you can just work on transitioning your son back to his own room, instead of getting them used to sharing and then having to change everything again. I hope I helped ...