Bigger forces!

Selene was kept under absolute surveillance. Numerous maids were appointed to make certain she doesn't leave the castle premises and stays inside the walls of the castle. Artemis involved her in ladylike activities like teaching her piano, stitching and embroidery, gardening, and other activities which she taught Ariel when she was her age. 

Selene followed her mother around like a little pup. She was obedient and never argued or tried to make excuses to somehow delay the task. 

But Selene wasn't going to stop by any means. She practiced in her room whenever she would get time, mostly when her mother was away. She knew if her father and brothers need help at any point of time, she must be serviceable to them. The last time she visited the training grounds, she stole her sword and sneaked it into her room somehow. It was quite obvious Artemis would keep her under watch soon so she made other plans to practice.