Is it weird for me to be unconcerned about my dad when he's practically dying?

So my life started out with both parents, but at age 11 my mother died. It was just my dad. He never remarried even though there were women in his life. My dad and I certainly didnt have the best relationship when I was a teenager, but he softened just a little bit while I was in college. I'm now 31 and he's has cancer in the thigh (he says the doctor says it is an unknown type) and has had surgery in the past 4 years, and recently has had chemo and radiation. I call him about once a week because he gets upset with me if I wait any longer than that. When I do call him, he tells me about the effects of the illness and the treatments. I cried one time when he told me his hair started to come out in patches. After that, I have been pretty unemotional and I really dont get caught up in what he's feeling. My sister stays 20 minutes away and she doesnt seem very concerned either. I'm not sure if its because our dad has a history of over exaggerating or if its just because he wasnt the best father when it came to our emotions growing up. Is this weird? If there's anyone else that's been through this and have felt very un-attached to a potentially dying parent please write back. I just want to know if something psychological is happening that I'm not realizing.