I've decided to get a tattoo, how do I deal with my family's disapproval?

I've fantasized about getting a tattoo since I was a little girl, but rejected the idea for all the classic reasons (risk of illness, taboo, regret, not getting a good artist or design, etc.). I changed my mind two years ago when I got an elaborate henna tattoo on my upper left shoulder that I loved so much I sponge bathed for a week to keep it from fading. Shortly after, I got together with a man who has a beautiful custom tattoo done by a local artist with an impressive portfolio. I've since been playing with a design that has much personal meaning to me (the Phoenix, to go on my shoulder), and now that I have graduated from college I've decided that I'm ready to do it. My maternal grandmother and my step-dad are very supportive and have given me money for it (graduation gift) and even want to sit with me when I get it. But my mother and teenage sisters are extremely discouraging, claiming that it's just a phase and that I'm doing it just to copy my boyfriend. They behaved similarly when I got my navel pierced at age 18, and my Mom still makes comments about how trashy it looks even though she never has to see it. I've decided that I want the tattoo, and am at peace with the decision and the knowledge that I'll have to keep it covered for my career, but I really hate to get it with so much bad energy around me. Has anyone else had to deal with this? Any tips for how I can put their minds at ease?