There is a another website, I think it is purina or pedigree, where you put in your profile and what characteristics of dog you want(size, activity level, hair legnth, where you live, etc). It is very good I checked it out even tho I am not really looking for another dog.
I understand what you saying about your sister being too young, some friends of mine went to their local H.S and they wouldn't let them adopt b/c they had a 5 year old son and she was pregnant!
I went to the HS looking for a small dog and came home with a greyhound!!!Which is a big dog with a small dog personality. They had all sorts of dogs there, long haired dashunds,boston terriers, dalmations,several huskies,rotts, GSD, dobs.
When my girl stands at my side, my hand touches her back, but she can also curl up to the size of a poodle when she wants too!(except when she sleeps!!!)
If you want a dog that is good with kids, moderate activity level, very loviing- get a greyhound!! BUT it helps if you have a fenced in yard, and I speak from experience here b/c I do not!!! And they are very prone to separation anxiety, as any shelter dog, I learned the hard way!!! But I wouldn't trade her for the world!!!
Whatever you choose, good luck, check out the rescue sites for different breeds, simply b/c they will list the +/- for their specific breed. I am sure we are all anxious to hear your decision!!
~~~~~Smiles~~~~~