Helping my mother cope with me moving across the country.?

I'm 27 years old and am moving across the country to live with my boyfriend. We've been long distance dating for a year now and can't bear the heartache of being 3,000 miles apart. I fear that my mother is going to be heartbroken. I have a younger sister who lives close by. My step father tells me that she cries all the time and is having a really hard time with this. I feel terrible. Everything seems to have fallen into place for me with this move except for this last piece. How do I help my mom cope? What can I do to make her feel better about this move?