Now you're shown us line 9 of the pattern I'm even more convinced that 'S' is a mis-print for '5'. Just like on line 10 you have 13 sitiches before the pattern repeat (between the asterisks) and 3 stitches after the pattern repeat. If you subsitute 5 for S in each line you have 25 stitches in both pattern repeats, which go exactly into the 100 remaining stitches your mother will be knitting.
An English Knitter (nationality and method).
Feel free to call me either Clutter or Julie (my real name).
A sweater of a thousand yards begins with a single stitch. Lao-tzu's Granny