Am I a brat? TW: mention of loss

My sister in law announced she was pregnant shortly after I had my miscarriage of my first pregnancy. A pregnancy we had had long wanted after a year and half of trying.. My sister in law is the sweetest girl ever and I was so excited for her if not still sad for myself but still just becuase sadness had happened to me didnt diminish her right to happiness. Well fast forward and she just had her baby. The baby is such a precious little thing and they make such a lovely family but all I feel when I look at the baby is deep, aching sadness for the baby that should have been just a few months older than she is. I feel so terrible and worry that this feeling won't go away. I have wanted to be an Auntie for so long and now I worry that I am never going to be able to get past this intense desire to sob everytime I see the baby. (I, of course, wait till I am alone and at home to do any crying and I dont say anything to the new parents about my feelings) I feel selfish and rotten for being consumed with thoughts of myself in thier joyful and exciting period of life. Am I a monster? How can I move past this?