I'm not going to my grandfathers birthday
Why aren't you going to your grandfather's birthday party?
I don't think that my grandfather really cares about me
You should take your family responsibilities more seriously and attend your grandfather's birthday party. You are behaving selfishly and your family will not be pleased with your behavior.
What do you know about my family or my responsibilities? My life is complicated.
I don't know much but I know that birthdays are important. And that they are a way to let the person know that you are glad that they were born. And this is your family, it should mean something.