I don't know what your soil is like, and it sounds like your family generates more greywater than the family who lived there before you. Essentially, your "perc" from your system needs to exceed the rate at which you can sustain output. The idea is to have your perc half of the system drain the cistern while you aren't generating greywater, yet have a cistern big enough to accomodate your greatest net output, minus whatever percs while it happens.
Get some suggestions from your local septic engineers. They know your soil conditions and local building and health department codes better than I do.