If they are Scotts, there is a high likelihood that they are Christians. The dominant religion in Scotland is Christianity, and the dominant Christian denomination is the Church of Scotland (which is a Presbyterian church).
All that, however, is also beside the point. No writer is obliged to tell the reader directly any type of information. Writers can imply information. By saying that it was not "Scottish Presbyterian way" to do something, the author is indirectly telling the reader what religion the protagonist or her mother is.
Of course, there is always room for interpretation, but putting all the (available) pieces of information together, it is likely that both mother and daughter are Presbyterians because they are Scottish, because most (although not all) children have the same religion as their parents, and most importantly, because the author specifically referred to the "Scottish Presbyterian way" to describe the mother's behavior.