I find it strange that she would all of a sudden adore her sister like that and want to follow her path.
Like she said, she barely knew her and it felt like she was a relative but all of a sudden she became a super groupie??
dont forget this girl was spoiled and had as much love as she could ever want, but the thing she needed and didnt get was her sister. Philia was kicked out at 5 and trained for 10 years, and Mia was shown to be 15 already when she debuted, so for pretty much all her life she never got to see her sister.
Then Philia is praised as the ultimate saintess, and when Mia snuck out to see her and almost got killed, coincidentally Philia was the one who saved her. To see the sister you've only heard rumours about and rarely get the chance to even glance at her suddenly save you from a near-death experience would be pretty fascinating, i'd bet.
Mia remarks herself she was a spoiled child, she doesn't know all that Philia went through as their parents kept it hidden from her. She just knows she went through "10 years of training".