Let your monthly data use be represented by X
Then:
- Your sister's use is 1/2X (half of your usage)
- Your father's use is 2X (twice as much as you)
- Your mother's use is 4X (four times as much as you)
The total amount of GB use in your family can therefore be represented by

This total GB use equals the limit of 6GB per month, so

Divide by 7.5

Hence, you use 0.8 GB per month
- Your sister's use is 1/2 * 0.8 = 0.4GB (half of your usage)
- Your father's use is 2 * 0.8 = 1.6GB (twice as much as you)
- Your mother's use is 4 * 0.8 = 3.2GB (four times as much as you)