I have a few suggestions to add to Elmy's excellent answer. In case you can only get a rough estimate of your sister's measurements, it will help if you choose a pattern that has some flexibility in terms of sizing.
The most extreme example of free-sizing is a wrap skirt. You can make a wrap skirt for someone with only a rough estimate of their waist and hip size. As an added bonus if you're a beginner sewer, wrap skirts are very simple to make. You can make them with all straight seams, which are much easier to sew that curved ones. Here's a link to a tutorial. Make sure there is plenty of room for overlap in the front, otherwise the panels will slide open while she walks.
A slightly less flexible option is a skirt with a gathered waistband with elastic all the way around. I recommend adding a drawstring as well, otherwise the entire skirt can be easily pulled down if the hem gets caught on something. A tiered peasant skirt is one style that works well with an elastic waistband. Here's a link to a tutorial. A downside is that this is not the most flattering style. It works best with very lightweight fabric; the thicker the fabric, the more bulky the waistband will be.
A more fitted and flattering option is a mostly-fitted skirt, with a small amount of elastic in the back of the waistband. That way the gathered fabric (which is the least flattering part of the waistband) is hidden in the small of the back, where it creates some extra room for the bottom, and in front you have a smoother, more flattering waist. One of my all-time favorite skirts uses this method. It's fits nice and snug, but it also stretches a bit so it's comfortable when I sit. Basically you just make a skirt with a larger waistband than necessary, and sew a piece of elastic across the back of the waistband. If your skirt pattern has a stiff waistband reinforced with interfacing, you may want to eliminate the interfacing on the back of the waistband. Here's a tutorial for a style of skirt that you could use this method with. The tutorial has you put elastic all the way around, but you could easily modify it to put the elastic only in the back.
All the above options will work with woven fabric, which is not stretchy. Woven fabric is much easier to sew than knit fabric, so it's a good option for a beginner. However, if you are willing to work with knit fabric, you can make a skirt with a stretchy waistband without any elastic. Here's a link to a tutorial. If you follow that tutorial, I would recommend adding an extra step to hide the fabric edges at the seam where the waistband attaches to the skirt. Otherwise those edges will be scratchy on the inside. Instead of sewing both layers of waistband to the outside of the skirt, I would only sew one layer on the outside. Then fold the waistband over, tuck in the raw bottom edge of the waistband, and sew it down by hand.