Mia was glad that Ashton didn't change his mind about pursuing Phoenix. She liked Phoenix, her intuition told her that Phoenix was a rare gem, and she was glad that her brother took a liking towards Phoenix.
As a good friend and a good sister, she had long decided that she would do everything that could help her brother to win Phoenix's heart.
So what if Phoenix had kids? Does Mia think that Phoenix's value lowered because of it?
Of course not!
From what she can see is, it just showed that Phoenix was a responsible person, and wouldn't abandon her responsibility even if things were tough. It showed that Phoenix was a strong woman and a great mother and she values her family.
Phoenix was just what? 24? 25 years old? and she has five year old kids. So that means she had them when she was 19 or 20 years old. She was young then, things could happen, but does she run away from her responsibility? Does she abandon her kids in an orphanage?
No.