Hi Bookmaven,
You don't have to say a lot to open the door with your sister. Just acknowledge that you did hear what she said even though at the time you minimized it. Tell her you have been thinking about it after watching your mom say what she did to her. That's all you have to do. You don't have to get into what you are working on if you aren't ready.
My grandmother was quite a bit older than her youngest sister. I used to watch them argue over their childhood memories. It came down to the fact they were both right. My grandmother was raised dirt poor by two drunks and by the time my aunt came along, my great-grandmother had stopped drinking or stabilized enough to hold a job which stabilized the family quite a bit. Everyone doted on my aunt as she was the baby. She had a pretty decent childhood.
Take care!