Feeling discouraged

Caitlin • 31 years old, married 4 years! We tried to conceive for 2.5 years. finally got pregnant and gave birth to a healthy baby boy Aug 10th 2016. Getting ready to try for one more here soon!!!
Found out today my sister in law is having a baby boy. I am so happy for her she deserves it however I also feel kind of sad. My husband and I have been trying for almost 2 years to have a baby and so far nothing. My whole life all I wanted was to be a mom and it feels like I Will never have the chance. But I have to put on this happy face for all my family members when it is tearing me up inside. (4 sister s in law that have all either gotten pregnant or just had a baby in the past year) they keep saying " your time will come" or "just wait till you have your own" don't they see I would kill to be in their position. The worst is it was so easy for all of them and they tell me that, if it's so easy we should have two kids by now. I feel so selfish feeling this way and have no where else to vent to so thanks for listening.