To measure her full grown height, there's an old trick to use, measure from the hairline of their hoof the the center, not the bottom, not the top, of their knee, however many inches it is is how many hands high the horse will be around (give or take .1) when full grown, my new yearling will be around 15.2
As a general rule [bearing in mind there ARE exceptions] a horse will be 90% of its adult height at 12 months of age. My calculations might be a bit off due to her being nearly 15 months but we'll see.
That sounds like a lot but a horse that's going to mature 15hh will be around 13.2 at that age. 6 more inches. And the bigger they are, the more of their mature height 10% actually is, so the more they grow after their first birthday.
So say she's 14.2 now, we'll figure conservatively just because. First we need to figure out how many inches that is. 14x4 is 56. Add 2 for the 2 inches more, 58. 58/9, to figure out 10% of mature height, [used a calculator for this part] is 6.44.
58 add 6.44 is 64.44 which is just about 16 hands and an almost half inch when you convert that back into hands high.
Depending on her growth patterns she may or may not make that height and she may or may not end up taller.
Easily within the realm of possibility given the information you've given us about her parents.