It would be easier for your SIL to measure a sweater similar to the one you plan on making that fits her well - getting the tape measure in the right parts on the body is trickier than it sounds.
You'll basically need the chest/bust measurement that's just below the underarms, the underarm to hem and underarm to wrist. Then the length of the armhole - that's not along the curve, but straight up from the underarm to the shoulder seam. This diagram
can help, though you may not need #s 2 and 3 or 8 and 9. That will all be easier on another garment that's laid flat on a table or bed.