Being a convert, I’ll tell you, one of the major things that turned me off before I was Catholic was when people would start with how important Jesus is in your life etc; thus, my first thought on this is to ask why - you need to understand what is going in in your sister’s faith journey before you can start to bring her back to Jesus.
So IMHO start with listening to her heart speak, try not to argue, try not to explain the faith to her (she may not be at a “place” to hear this anyway), ask open-ended questions to get her to explain her logic and her emotional reasoning.
once you have this information, walk away for at least a day, think about what was said and then either ask for guidance from your Pastor, spiritual advisor, or even come back here and seek some possible ways to respond.
throughout this whole process; faith (pray for her and the children), love, charity, and patience should be your tools.
The sister is Catholic already. I would be very careful about presuming to tell her that I was there to “bring her back” to Our Lord. That really smacks of “
I’m there, while
you are obviously not,” which can be taken as a modern version of “Thank you, Lord, that I am not like other men…” Remember that the Pharisees were chosen as examples in the parables of Our Lord not because they were the worst sinners, but to demonstrate very clearly that knowing about God and doing all the right things is not the same as a humble relationship with God. (Some of the Pharisees did get called out for their moral abuses, but not every single one.)
No, I think I’d recognize the good intention in her reasoning for not having the child baptized (whatever that is) or the reality of the challenge she feels is in her way (whatever that is) but then also present the faith as a treasure that the child has a right to have in the same way as the child has the right to have other advantages in life, whether she likes it or not, such as good manners, the habit of gratitude, a work ethic, financial knowledge, the habit of recognizing the feelings of others, the academic skills, and so on.
It is all about giving the child the best opportunities in life, after all. The child cannot forced to believe any more than the child can be forced to be a scholar, but being given opportunities in life is what all parents do for their children, even when the children aren’t always wild about it. If the parent fails, the child has a legitimate gripe against them.
I do know people who feel their parents let them down by not giving them any religious upbringing, if only because it is a major chunk of cultural knowledge. It is embarrassing to celebrate Christmas and not know the Christmas story or to be asked a question about your parents’ religion and to not know the answer or to hear allusions to Biblical figures and to have no idea who those people are. Ignorance is no gift, even though receiving knowledge requires work in the recipient.