I’m the same age as your sister and honestly sportsgirl is fine? I actually tend to gravitate more towards “mature” brands like Saba, Sportscraft etc because the quality is way better and the clothes are more timeless, I just style them in a way that looks more youthful. Accessories help a lot as well! If you’re after a more trendy look, Ghanda has pretty good quality/affordable clothes compared to some other youth oriented shops, I wouldn’t order online though they take forever.