Choice defines immediate family as: "spouse, partner, children, parents, in-laws, brothers and sisters" for purposes of award stays. I would say a niece could be an in-law in at least some circumstances.

That said, not all relatives have the same family and/or last name, and I've never been asked by Choice to document family ties when the last names are different. The definition of "in-laws" is sort of subject to interpretation and seems to be a relatively broad category, especially in a world where it is possible to have in-laws even in the absence of having a government-recognized marriage involved.

What I do more frequently is book the room under my name and put in the guest's name in the special requests/notes area as the person who will be checking in. I've never had a problem because of that, but I do suspect that it is in some ways at least marginally a bit more risky than having the reservation set up over the phone by Choice.