lad, I think I may have your answer for you.
First of all, I think you need to understand something. If she was going to lie to you (about really not wanting to go with you) I highly doubt she would use the excuse of 'having to ask her mother.'
I am going to ask you to do some research first of all.
1) What religion is she ?
2) Of what culture is she ?
3) Has she ever had a boyfriend before ?
4) Has her brother ever had a girlfriend before ?
I ask these questions because, maybe this whole thing is a cultural issue. It sounds very familiar to me, because I was once in the circumstances you are in. Yes, she was 20+ and yes, she REALLY did have to ask her mother's permission. A lot of cultures don't allow a girl (or boy depending, however usually girls) to date before they are serious about marriage.
You said they have a family-run business ? Another sign of over-protection by the parents for the reason of attempting to stop assimilation of their children by the general culture.
I say, don't take it personal until you are sure of what the circumstances are. It could be the total opposite of what you expect. Maybe she REALLY does like you, but not enough to flip her house upside down to see you (due to family/culture values).
And also, you said she was a very religios girl. This may tie in with her culture.
Snoop around, ask some questions.
"I still find each day too short for all the thoughts I want to think, all the walks I want to take, all the books I want to read, and all the friends I want to see."
- John Burroughs