Pregnant, overwhelming sibling

My sister is 7 weeks prego! Yay! However, she is super moody and complains alot. She is graduating college and is 7 hours away. She wants me to go and I'd love to, but I have work and school starts back soon. She keeps on and on about graduation and where I am going to stay when I get there (hotel or her house) I really need to focus on work and school instead of making the drive and staying for two nights and driving back and going to school. Am I being too harsh? Is there a way I can say it without her being upset? Again, she is super moody! Thanks!