Firstly - His sister saying that you have ruined his life....well it takes 2 to make a baby sister!!!
Secondly - At just 18 when I fell pregnant, it was with a policeman (10 yrs older than me) who I had known for about 2 months only! As soon as I told him he made it very clear he didn't want anything to do with our baby and she is now 17 months old and he has never seen her. He never told his parents / sister and they only know now because I tracked them down 6 weeks ago and told them.
Point of the story is.....if his parents dont support you and his sister doesn;t like you, who cares?! Be strong, i am sure even you know you could do this on your own if it came down to it...I did and still am.
I am certain once their grandchild is born they will be the first ones knocking on the door to see him / her. It might not seem real to them yet but once it sinks in and they realise this baby isn't going away, they will come round. If not, it's their loss not yours.
And if your feeling really strong then rise above it, be polite to them, dont make any comments back and bite your tongue....how can they hate you if you are only ever polite and nice to them?!
Just an idea, good luck tho....hope it all works out for you xxx