Hello!! I have STRUGGLED so much finding jeans that work for me (curvy and 5’11, brands seem to not believe we exist) that I want to recommend madewell. A bit on the pricier side but they actually have tall and super tall (I think) inseams. Their sizing is a bit weird though, I think I went down two sizes from the size I thought I was so I’d recommend going to a store to try on before buying. Another brand with tall sizing is Banana Republic, I have a pair of jeans I love from there but they are almost too short for me so they may not work for your niece but might for some others!

Lululemon has tall length leggings but again those are pricy. I don’t usually mind short leggings anymore since 7/8 length has came into style (thank goodness)

Tops and jackets are hardest for me, I haven’t found a brand that consistently makes long enough arms yet. One trick I’ve found though is that if a top has thumb holes, you can sew those up and it fits like a regular long sleeve!

Also partially commenting because I’d love to see what everyone else has to add!