Dear jules32,
sorry to hear of the loss of someone in your life. Before doing something talk to your parents, a reverend, pastor or church elder about how you feel. I know it's hard, but these kids have it harder now and you needn'd want to visit them and hurt them more with talking about it with them until you understand how you feel. You can always lend the christian hand of samaritin, especially during this time of year. Offer to babysit, clean house, ask your parents to teach you to cook something and give it to them, help with homework, spend time playing with them or just listening to them if they need to talk. The young one will never know many things, you may even pass to college before that one knows about mom. The older childern will feel many emotions about the loss and they will express it in many forms. Anger is certianly one of them. They have a long road of growth to get through the loss and accept that they are ok. This may take years, but it's not your job. Just let a parent know if one is acting out terribly, they may need help that you cannot provide. And that is ok also.
Remember this, god doesn't punish us, he is beside us bearing our pain at the same time, we are never alone, we just need to open our hearts and listen to him. God was there when the accident occurred and recieved their mom with open arms. God was also there when the youngest was born and brought joy to their family.
Bless you for thinking of them,
m6