Am I wrong to feel like this?

About a year ago when I was 15 I thought I was pregnant my period was late and the test even said I was pregnant but than I got my period . Ever since than I've wanted a baby. Don't give me the comments about how I'm not ready and all of that . I'll admit last year I was not ready but now I am . Me and my boyfriend of 4 years are trying to have a baby. He is 18 has his own apartment and has a good job . I've been taking care of my niece since she was a baby, she is 17 months now and I do everything for her because I live with my sister and her husband  right now . I just found out my sister is pregnant again and I'm happy for her but at the same time I am so jealous and I feel upset because I wish it was me ... I've been so depressed about not being pregnant . Honestly I don't know what to do .