If it isn't too personal a question for an open forum....... how old are your little siblings? (I only know the age of the sister that you are always asking prayer for.)
I think it is great that you want to take an active interest in your younger siblings. Reading a bedtime Bible story and praying for them is a wonderful idea. You could offer to help them with homework. Pray with them before test days. Take them to the park. Play with them. Drive them to any activities they are in and stay and watch. There are so many things you can do as a big brother that will show them love and provide them with an example of how a man and father should be.
Just don't be in too big a hurry in your own life to get married and have children. In God's perfect time it will all come together. As you spend time with your siblings, think about it as God's training for your future. It is actually much easier to be a big brother than to be a father. Take joy in your opportunity to form a loving bond with your siblings.