How many spools of thread did you use to quilt it? Now, how much do those weigh. Even ON the spool you are talking about negligible ounces of weight from the quilting. However, the batting, and the piecing, as others have said, will play a major role. Also, the fabric itself. Some cotton is lighter weight than others. I made a lovely quilt with shot cotton and quilter's dream select batting. It is the lightest quilt I own, despite being a full size quilt.
And yes, I completely understand how a quilt can be too heavy for someone as they advance in age. My mother has several quilts I made - only one is light enough for her to manage now. I don't think you are wasting your time at all, especially if your sister uses a very lightweight batting.