...what about the posters who have said it might be a relief to have it in the open? That is all I am thinking here. That it's a hard subject to bring up, that perhaps they might appreciate their baby being acknowledged, that it might bring some comfort.
What makes you think they want comfort?
They moved on. They had you. You have not related any indication of trauma about being parents on their part when you and your sister were under their wing. The baby has been talked about even if briefly.
The idea that your mother needs comfort and that you are going to deliver it, out of the blue, is self-serving.
What makes you think you are the person to deliver comfort, or that now is the moment when your mother needs it, or that your mother has not peacefully closed that chapter in the book that is her life?
This is not about comforting someone else. It's about your perception of a snub, your need to feel close to your mother, your need for an explanation of why your mother kept a memory to herself.
Your mother never had to share more than she felt like sharing with you. Try not to see her decision to keep some things private as a poor reflection on your relationship with her, or as an indication that your relationship was not what you thought it was.
Did you get any help processing the death of your dad?
I've had four MCs. Three were under 9 weeks, one was at 13 weeks.
I don't like talking with my DCs about them. They were traumatic at the time but not any more. I have also lost a sister (in adulthood) and I don't like talking to my DCs about her either, about her life or her untimely death. Even though my DCs are all adults now, I prefer to keep some memories strictly to myself. Her loss affects me in different ways as time goes on and I don't feel as a parent that it is right to unload that on my children even though they're adults. I haven't kept things bottled up in an unhealthy way that needs addressing - I have had counseling, and I have dealt with the gap in my life in different ways that have worked for me, including many deep and meaningful conversations with my other sister and my mother over the years. My DCs' were too young to remember their aunt, and trying to convey her to them in words would only serve to reinforce the finality of her death and the distance that separates us all from her.
I think you should consider the possibility that trying to get your mother to talk to you about the baby who went before you would not be as helpful to her as you want to believe it would be.