Do you talk to your mom? Dont you think you should try and have a conversation with her? I would encourage you to speak to her, you may be pleasantly surprised by what she says. Sometimes, we assume so much about others.
Its not the end of the world, just try to work on the relationship between you and your mom. Just becoz she is having another child dosen't diminish your worth. Try to take a positive look at the whole situation. Wouldnt it be nice to have a sibling who you can spoil and love?
I dont understand what you meant by her disowning you. Did you do something she felt was wrong? Even if this may have been the case, its good to move on and forgive. And if you did do something, did you make sure she knew you were remorseful?
My advice is to try and look for positives in your life, to reach out to her and speak from the heart, open up and let her know what is going on with you and how you feel. Should she choose not to have a relationship with you, dont despair, just let her know that the door is always open and you will always love her.
Go ahead and give your best in school and everything else. Do you know how blessed you are to have the ability to go to school when many kids in third world countries long and wish for this opportunity with all their being? They have to work and provide for their families at young ages with grim futures.
Do you know how blessed you are to have a mom? How many wish and long for the opportunity to just have one day with the moms they have lost.
Do you know how blessed you are to have good health, food or shelter? There are kids who have to wander from place to place and dig in trash for their next meal with no clothes on their backs.
There is so much to be positive about and be thankful for should you choose to look around you and at yourself.
Strive to be a good person and a good daughter no matter what the outcome between you and your mom. Be happy and be ready to be a big sister.Life is too short to be depressed and to miss out on all the blessings we are granted. :)