Anonymous wrote:OP here.
Actually, I should clarify: I already asked, but in a casual way. She had offered me the dining room table (also in her basement) which I turned down. Then I noticed the sideboard and said, "Are you giving that away too?" and she said no. So if I ask, it will likely be seen as pressing the issue. For what it's worth, my sister and I have a pretty prickly relationship. Sigh.
You asked once, she said no. Mentioning the issue again *WILL* be pressing the issue. You didn't mention in the OP that you had already been asked and denied. In this situation, you move on and find something else. You can revisit the issue again in the future (at least a year away) and then try to ask nicely.
You don't know if she already has plans around that. For example, although she is moving into her bf's fully furnished house, she might want to try to move that into the dininr room after she's there a while. Or she may have another room that she is thinking of redocorating and may want to use the piece.