Why is it always someone else’s turn

Today is little harder then some. We have wanted a second baby for so long. Yes I’m blessed to be a mother once and I am thankful to god every day and count my blessings however we have wanted and prayed for another baby for so long. A tiny little one to add to and complete our family,to make our amazing beautiful daughter a big sister which we know she’ll be amazing at. Today was a little harder cause my sister found out she’s pregnant. She didn’t want anymore and was done. Part of me is so happy for her and I can’t wait to meet this new nephew or hopefully niece but the other side is jealous! I wonder why? She hasn’t even been trying. I feel like every time I turn around a family member or good friend is getting pregnant. I’ve been trying on and off for four years. I just try to keep in mind god has a plain for me and my fiancé and hopefully it’s another little one. Anyways needed to vent. Thanks for listening.