"...what things the sister actually likes to do." There is no more important thing than knowing this. What will this woman-to-be do with this math? Will she be

"...what things the sister actually likes to do." There is no more important thing than knowing this. What will this woman-to-be do with this math? Will she be an business manager or an accountant? Will she be a fashion designer or a fabrics engineer? Will she be a naturopath or a chemist? Each of these either/or's has a significant difference in the amount of basic math required. Once you discover what she loves and is naturally good at, help her follow her path, and then add in math as necessary. Honestly, there are some people for whom cross multiplication will never factor in significantly in life. You may be wasting her valuable time by pushing her in mathematics.