I am sorry you are going through this. You did nothing wrong, and no amount of prayer makes babies. God is not a genie. What kind of God would just pick people that met some arbitrary level of prayer and grant their wishes. Prayer whether you chose to use it or not is there to give you strength to get through lives ups and downs. So, please know you did nothing wrong. Life can just be shitty sometimes, there is no reason for it.
I'm so sorry you're hurting and REALLY hope that this is YOUR cycle and you can share in this joy. I really think your sister's being an asshole though and I would like to smack her with a herring.