it depends who built the time machine.
if i built it, went back and killed my grand father, i couldnt have built the time machine to go back, to kill him. so me, my grandfather, my father and the time machine would wink out of existance.
as none of us exist, my brother woudlnt exist because my mother would have met someone else. etc...
if someone else built the time machine, but i went back in it and killed my grandfather, we would wink out of existance, but someone else would go through instead and kill someone....
it would change alot.
but personaly i wouldnt kill any one, id go back and meet myself, tell myself what happens, so i can relive it all, but better.