In order to take Weapon Master pre Epic, at the latest you must take levels in Weapon Master is at level 16 (as levels 16, 17, 18, 19 and 20).
Your character is level 12. She will get one more feat at level 15 (before level 16).
That leaves you two feats short for Weapon Master at level 16.
Therefore it is impossible for her to take Weapon Master pre Epic.
In fact, being three feats short at level 12, she wouldn't be able to take Weapon Master before level 22.
Hope this helps.
Regards,
Script Wrecked.