A good location portrait photographer takes each location on it's own merits. You don't always have to think "full pack" in terms of the backdrop and all three lights. Nor do you always have to use the backdrop for a portrait either.
Sometimes, one monolight will work fine for a family portrait--especially if lack of working space is an issue. Many times, the natural light in the room will work just fine. Also--and speaking of "room"--sometimes the room itself will make a great background itself (depending on how it's decorated and such), and at times eliminates the need for a backdrop.
If space is a consideration, consider using the client's garage. You can also shoot outdoors using your stropes in "open shade." And speaking of "outdoors"--I've been photographing many portraits of families using the family home itself as a background; with the sun slightly behind the subjects and using my battery powered (or AC powered) flash equipment as fill.
If space is really limited--so much so that you can't set up a monolight with an umbrella, you can also use your shoe-mounted flash bouncing the flash off a white ceiling (if available) or use a dedicated cord to shoot off camera. Pocket Wizard Transmitters can come in handy also.
Remember--just because it's a family portrait; don't always get "tunnel-visioned" with always the background and all three lights. From a logistical point of view, I always start with the fewest amount of lights I can get away with and add from there.
Everyone has seen the family portrait in front of the backdrop. It's cliched and expected. However--if you can add good variety to your family portraits, giving the customer more choices (from what they see all the time), you'll find your customer base will increase.
Just my professional two-cents. Good luck.