Autodesk SketchBook might be a bit more popular than Paintstorm. We know about 1 link to it since March 2021 and only 1 link to Paintstorm. We are tracking product recommendations and mentions on various public social media platforms and blogs. They can help you identify which product is more popular and what people think of it.
If she wants to expand and more than just mimic traditional paints there are even more programs she might want to consider, the king of the "generalist" softwares is Photoshop, my personal choice, but there are valid alternatives out there, starting from the free and open source Krita, to cheap but solid programs like Clip Studio Paint (especially good for comics/manga, but it can also do other stuff), Paintstorm... Source: about 2 years ago
Try sketchbook.com, the official site. I don't know if it's different from the Microsoft store version, but it might be. There should be a free trial. Source: about 2 years ago
Krita - Krita is a professional FREE and open source painting program. It is made by artists that want to seaffordable art tools for everyone. Concept art. texture and matte painters, illustrations and comics.
Corel Painter - Corel Painter is media painting software.
Procreate - Procreate is the most powerful and intuitive digital illustration App available for iPad®.
Clip Studio Paint - The artist's software for drawing and painting.
PaintTool SAI - Learn to draw in different ways.
ibis Paint X - Drawn with + ibisPaint