What is the etiquette on borrowing things?

I lent a friend some books, two of which are my sisters. I dropped them off to her and she’s had them for awhile now, and my sister wants her books back, as I want mine back too. I’ve asked the friend when I could get them back, and they said by the end of the week, but that was almost two weeks ago now and she hasn’t brought them back.

Is it wrong for me to expect her to bring them back to me? Or should I be the one going to get them??