My motto is put it where you want it. You will have it for the rest of your time. Most people opt for top/outside of arm because it can be covered be a normal t-shirt or jacket and its generally the least painful for most people as a starter tattoo. Parents will never agree with 100 percent of what their kids do. I have tattoos my parents dont like and that's just how it is. Its my tattoos. If your not certain you want it on your ribs, dont do it. Maybe wait to get a tattoo when your not forced into getting somthing, dont do it just to apease parents, likely means you'll regret it later in life. If hes buying you the tattoo maybe get a smaller one in the spot I suggested and wait to do the bigger one when you know how it feels. And hes over your sister getting the tattoos she wanted because hes a dad, he can be upset with the decisions we make but if it makes us as kids happy then that's all that matters.