you did say portraiture.. and you also mentioned a budget. Within those parameters, the Sigma 50mm 1.4 is probably your best option. A zoom will allow some versatility in composition.. but there are no zoom lenses in your price range that will produce pro IQ. If you increase the lens budget to around $1500, you might be able to get a decent zoom.
if you are planning on doing this professionally (which is what it sounds like) then you need multiple good quality lenses, multiple flashes, radio triggers, lightstands, reflectors, possibly monolights an diffuser systems ($10,000 would be a good start)... and a lot of experience. You can buy most of those items.. but not the last one.
Family portraiture can be very difficult.. you have so many variables. Different subject heights, weights, shapes, shades, colors, luminescence, reflectivity, DOF, and attention spans. Moving this outdoors multiplies the issues.. with harsh light, flat light, dappled light, and light of many different shades and colors that you have to compensate for.
You will also need to be very good at some editing package.. preferably Lightroom ($200-300) and / or Photoshop CS5 ($200-$600).. to help compensate for errors you make, and the inevitable goofs the subjects make.
Have fun....
(P.S. don't forget legal liability, insurance, bonding, contracts, and a good business plan)