Glimmer probably knows Tam's last name, and that's why she's so nice to him and trusts him. Glimmer might have gotten kidnapped when Tam & Linh were born, that's why Glimmer wants to be friends with Tam, she wants a better relationship with her brother.
She might have also ran away because her parents weren't being good parents. And the Neverseen taught her how to use her power and/or triggered it. But then she realizes that the Neverseen are actually bad, and goes back to live with Tam & Linh.