How do I say no to my sister spending "a few nights" in my apartment?

Depends a lot of the type of relationship you have with your sister. Also, I think you should consider why she wants to "spend a few nights" with you. Is it just for fun? Does she have money problems? Is she escaping from an abusive relationship? If you have already decide to say "no" it would be best, if it's culturally appropriate, to be open and honest with her. Perhaps you need some time to yourself? Or perhaps you know from past experience that her "few nights" always turns into ther staying for a few months! I hope you can figure out a way to talk with her and reach a good conclusion.

It would be really lovely to have you to stay with me for a few nights, but right now I am needing to work doing online teaching at home and so don't have space for another person, so I am sorry I have to say not now. I'd be open to having you to stay for a weekend or a few days when the pandemic is over and we are all much more relaxed and can return to some kind of normal life. You are really important to me, and I value our relationship hugely. How would that work for you?