I am so sorry you lost your Mom and especially at such a young age. My Mom is very sick right now with cancer and I know that the outcome will not be good and don't even want to imagine life without her!
Stay close with your family because in this life it is your family who will always be there for you and who will always care about you more than anyone in this world.
Your siblings are younger and will need you to remember Mom... your responsibility is not to replace Mom, but to help them remember her and become the adults she would want them to be. You can't give up on your own life, your Mom wouldn't want that. Live, Laugh, Love. Time seems to make the pain a little less, but you will never get over losing a parent.
Take care of yourself and your Dad and your siblings and try to always be there for one another and remember Mom!