Am I an underpaid nanny/assistant?

Hi, I'm a non-live-in "nanny" in the Somerset/Hunterdon county area of NJ, I have worked for the same family for about 2 years. I put nanny in quotes because I feel I do so much more than what is required of a nanny. I run errands almost every day (even on my days off, or before coming into work) , I help the mother out in her office, I iron the family's clothes daily as well as some light housekeeping. The children I care for are between the ages of 12-14 (there are 3 of them), so it is less "babysitting" and more driving all over to accommodate their hectic schedules as well as help them with homework and anything else they need (despite their age they require a lot of attention). The family is usually very kind to me, and has basically accepted me as part of the family, however I am beginning to feel like a slave. I generally work a 10 hour day 5 days a week (except during school, when i work 4 days at up to 12 hours a day). I looked up the average pay for a nanny in my area and I am paid on the higher end of the average at $15 an hour, but I feel that I do so much more than what is required of my other "nanny-friends" who make around the same amount. I don't want to seem greedy or lazy but does that sound like I'm underpaid? Any help is appreciated :)