If Xian is the non religious form of Christian (like Xmas means someone celebrates Christmas but not in a religious way) then I'm completely and utterly bewildered and confused what their stance is. I understand 'cultural' holidays of all religions. Cultural church going just confuses me.
I don't mean to sound ignorant, just never heard of Xtain or the religion they follow before... feeling like a fuddy duddy 
If what the teenager is a part of isn't 'just' mass, showing up late is rude in all the obvious ways I'm sure OP doesn't need explaining to her, like 'so and so's time,' or ''you signed u for this, be responsible!' or whatever.
Taking her halfway through a ceremony (if we take religion out of it) because Heaven Forbid her older sister was gasp easrly for work is breathtakingly rude. The sort of thing other parents go ]hmm] at.
and I say that as someone so lapsed when with my parents I have to struggle not to be rude. But I'm not there to participate.
TBH OP, if your DD is supposed to be a part of the ceremony and you'd rather show up half an hour late, clearly disrupting (and yes, it's disruptive - I remember from my indoctrinated days or worse, possibly upsetting) others, then your children need some manners, ASAP.
If you've done the late thing, then be prepared to be an item of scorn and gossip. I hated my parents faith, but even naive me would have viewed that as very, very very rude.
If you're okay with being seeing that way that's obviously fine. But I would hate for my child to spoil someone else's event (EG if they were speaking when you walked in)