About the luggage, it would be 4 large checks and 4 carry on, and we just got back from a domestic trip; our 4 carry-on would have almost filled up the trunk of a Sonata rental. I think if we have 8 suitcases it wouldn't fit at all (we also need 4 passengers and 1 driver).
We live near a CalTrain (walking about 10 mins), and Google Maps said if we take Caltrain to Milbrae then BART to SFO would be about 1 hr 10 mins. So it seems like it is the best option so far. Maybe I can drive to drop off the family, then another trip or two for luggage for them to watch, then walk to Caltrain after parking the car at home, which would be about 1.5 hr each way. It would be easiest not to ask a friend to drop us off in 2 trips.
The idea about renting a car from the airport, no, it is more hazzle and it would be even more expensive. This actually inspire me about using the airport luggage storage, maybe one of us can be there to either watch the luggage or store it at the airport storage ahead of time. It's $6/24hr so if we drive there ahead of time to store 4 suitcases it would only be $24+gas for a trip to airport.
Come to think of it, if we have 3 check bags and 4 carry on, life would be easier as I can drive the family, drop off 3 members to check bags, wait in line, then drive home and take the train with 1 carry on bag, and by the time they are done checking bags I can use my own boarding pass printed from home to walk through security, without wasting too much time.