I really need some advice..

My sister said she is 100% positive that she wants me and my husband to adopt her baby... but she’s not sure how she’ll feel after she gives birth and she doesn’t want to hurt our feelings if she changes her mind at the last minute.. is there any advice you could give for me to tell her? I’m not really sure what to say to her without sounding like I’m begging for a baby.. and any advice you could give me if she does change her mind? I know I will be absolutely crushed and it would honestly be a hard one for me to come back from because I have been ttc for 3 years and my sister has actually offered to do this before with her youngest son, but things fell through because she refused to have the baby’s father involved.. legally, both biological parents have to sign over rights. We don’t have the issue now, but I’m still so worried about her giving us this precious gift and then ripping it away from us.