to your original question, if you are going to use live rock in your 20 gal, the filter you have is a good start, but you'll need at least 2 more powerheads to provide current. as OCCFan023 said, read Lordroad's sticky on the nano page, its great.
I must also second what others have said here, if you don't want a saltwater tank, explain this to your family. it isn't that much work or difficult, but it takes time, patience, and most importantly, you have to care. if you really have no interest in the tank, you will be unlikely to stay on top of the needs a saltwater tank has. for that matter, a freshwater is unlikely to be much different. setup and care for any tank that YOU want to do, it should be your hobby after all. if members of your family really want a saltwater tank, then have them do the research, setup, and maintanence.
sorry to hear of your uncle's loss, but I hope that isn't the sole deterant to you setting up a SW tank. as Mushroomman said, if you take your time, do the research, and plan ahead, its not much hassle at all, no more than a FW.
good luck!