It’s good of you to want to help your sister, but there are so many erasers and pins of different lengths and we don’t have a scale picture of your eraser and pin. The way to do it is to a get a ruler and place the objects alongside the ruler so that the end of the object is lined up to the start of the first measure. Then read off the length by observing where the other end reaches to on the ruler. Make sure you use the side of the ruler showing inches. (Most rulers have inches on one side and centimetres on the other.) You can use a tape measure instead of a ruler, but keep it taut. Each inch should be subdivided into two halves and each half into half again (to make quarters, ¼, ¾, and two quarters make ½). So you can measure to the nearest inch, half or quarter.