My 4 soon to be 5yr old wants a sibling

My step daughter is the cutest! I just love her... I've been in her life since she was 2 and she's now just recently starting with this sibling talk. She'll tell me wants a baby sister sooo bad so she can have someone to take a bath with and play with. She says she'll be the best big sister ever and share all her toys. She was even telling me how she was gonna move her bed to make room for sister's crib 😭 Her mom is currently in a relationship so I told her well maybe one day mommy will have a baby sister and she says no I want her in ur belly and I just died! I hear all these stories about children knowing when u are pregnant and I'm waiting and praying she'll be the one to tell me but who knows. Her father and I have been trying for a little under a year. I'm praying every day I can make this happen for her ❤