I lost my mother in 1990 and my father died a month before my wedding in 94. We had difficulty conceiving and thus it was not until 2003 that we finally had children (and we had two sets of twins, so our cup runneth over). Even though it has been a long time, not recent like your loss, the emotions are much the same. I knew when I married that my parents would never know my children, and my children will never know my parents. My husband's father is deceased, and his mother does show a little interest in the children, but not really that much. I have 4 siblings that live in the same metroplex, but they have never even made an effort to meet the last set of twins born in 06 - you know, everyone is soooo busy - which I do understand since I am totally swamped all the time.
The loss of a parent is a major event in one's life. It goes beyond missing the parent's love and contribution - it changes the whole universe of your family - your role is changed because the matriarch is gone. All your life you have turned to your mom for guidance and companionship and now that has had to change. I am a probate attorney and I work with families who suffer loss on a daily basis. I chose that area of law due to the loss in my personal life. I highly recommend to you that you look for grief counseling, perhaps through your church. I really benefitted from counseling. Your mother's death is affecting you much more than you really comprehend.
I can tell you that with time the pain lessens. Your sadness will eventually be replaced with fond memories. God is giving you a second child and you will be so busy that it will be hard to have time or energy to think about your mom - I know you think that's not possible but just wait until you have two little ones. They will bring you so much joy, too, that it will help to mend. That is the cycle of life.
I have cultivated friends that sort of take the place of grandparents. Also my father had a much younger brother and I have recruited him to be a psuedo grandparent so that my children will know that side of the family better.
Good luck and take care of yourself. And check into the grief counseling class; it will help you much more than you think. Grief never goes away completely unless you deal with it; it just cumulates.