I'm not sure if retro games are a good starting point for children to be honest. I understand the irony in that retro games were aimed for children, however, design wise it was the wild west and majority of them can be somewhat obscure in terms of what to do and why. Since children during the 70s, 80's and majority of the 90s did not have internet, it was either keep playing to figure it out on your own, word of mouth or play outside lol
In the here and now, however, children who have access to the internet have an incredible amount of alternative things to do, which I think contributes to the lower attention span and threshold of patience if something becomes difficult or unclear. This will no doubt occur for your little sis if she is exposed to the wrong game.
While I don't discourage introducing your little sister to retro gaming eventually, I think there are better modern games for you to introduce her to gaming as a hobby to get her acclimated. But first, you should figure out what her interests are. Does she like gardening, nature, puzzles or designing? With that information I feel we can help with some suggestions that she is likely to find interesting.
