It sounds like you will make a wonderful aunt for your sister’s new baby! My advice to you would be to spend time babysitting and taking care of your niece or nephew. The idealized version of a baby and an actual baby can be very different. This might change your mind on whether you want to have a baby in your life right now.
As far as you wanting to have a baby it seems like a normal feeling, especially after your previous miscarriage. However I would wait before trying to get pregnant. I would make sure that there was a strong relationship with love and especially commitment before trying to have a baby. Would your boyfriend be willing to marry you? Do you love him enough that you would be willing to marry him?
If the answers to these questions are yes, then I would wait until you both finish high school to get married and then start planning for a family together. If the answer to one or both of the questions is no, then I would wait to try to get pregnant until you find the right person and can have a child in a loving committed relationship.
Kate