You cant force her to be involved, you can only encourage her to be. I wouldnt push the issue to hard because you dont want her to feel like she has to be involved and such which will only make her resent the baby.
Try sitting down with her and talking to her, she may feel like she isnt wanted around any more.
How far along are you? If you aren't showing or anything then she may just forget from time to time that you are pregnant? Men are the same way...they don't acknowledge anything until there's a huge belly or until the baby is actually born!
I wouldn't worry too much about it. If she loves her little sister then she'll come around and hopefully be a great help when baby is born!
I agree with the ladies, my own 8 yr old is really excited about the baby coming, but only if I actually ask him about it and what he might want to help me do with the baby, the rest of the time it is forgotten about (I'm 40wks and huge, but he still forgets!) Kids have too much else going on in their heads to think about us! My partners son says he is excited too but wont touch my belly or really talk much about it (he is 8 too)but I'm confident that both boys will be delighted with their baby sibling when he actually (finally) arrives!
Dont pressure her, let her be as involved or not as she wants let her make the decisions as to what she wants to be involved with and maybe make sure she's nothing worrying her at school or anything that might be distracting her if you think it is out of character for her to be so distant. I'm sure all will be well in the end :)