I'm not sure what the religion is... but I know a few girls at my school who seem to be in the same position as you!
Ask your mother
why it's part of your religion. I argue and question Catholicism all time time... I'm guess this is part of your "man made" religion.
For example, in my religion, only males can enter the priest hood. I've discussed this with my youth director just for curriosity's sake. Why can't women be priests? No where in the Bible does it say that women cannot be priests - it's just the way it is, and it's society.
A lot of the Catholic (and I'm guessing Christian?) sacraments are
man made, as in, man on earth came up with them, and God did not.
I'm guessing that 'your' God did not say, "I here by oppose women wearing slacks!" I bet it's just traditionally what women are meant to wear, and your church says that's right.
So ask your mom
why you're religion outlaws females wearing anything buy skirts, and you'll get an answer like, "That's just the way our religion is!"
If you see that crack in her armor, use Vadermort's suggestions and give her
real reasons that pants should be acceptable.