Depressed when baby is born?

Has anyone ever felt this way before? My brother just had his little boy today and I was a little depressed when I heard it was born. I didn't really know why but then I remembered that every time my sisters had children, I was a little depressed too. I am sad when I find out they are pregnant, but I'm depressed when they finally have the baby! I think it's a subconscious thing...my sisters/brother married into families that I didn't really like, either full of whackjobs or alcoholics. I guess at first it was a subconscious thing...but I am depressed because my family marries into these other families and I fear that the kids will turn out messed up or something....idk maybe it's not that bad, but I guess I was used to having the perfect 4-child family and no strings...and seeing as I'm the youngest in my family (I'm 17) it's always bothered me and I've always been sensitive. has anyone felt depressed then their siblings have a baby? o_O