I think it might be abit deeper hun then her being pregnant - dont get me wrong, its extremly hard at the start of the pregnancy, as you body has to get use to this "thing" thats taking EVERYTHING from you! And you wanna shut the world out - but at the same time, u still need hugs and kisses etc...i felt quite alone, so i needed extra company with my parents, boyfriend and friends!
From what you have said it does seem like its abit deeper - wont her sister tell you any more, as to do with what her problem is towards you?? did you live together?? You need to get this sorted before this baby arrives, otherwise its unfair to bring a baby into world war 3.
Just keep trying to talk to her - ask her mum, her sister. Do anything to get ur answers.
Good Luck x x x x x x