The couple should do whatever they want. The difference between a "ring bearer" and "junior groomsman" is only in people's heads, as is the idea that a ring bearer must be an adorable and utterly unpredictable preschooler. Or the idea that there must be a ring bearer at all. Like you, we didn't have one either, and no one thought it odd that there wasn't one.
It amuses me that folks insist "the ring bearer must be a boy no older than X, the flower girl must be a girl and no older than Y" and so forth -- after all, I've read about weddings where a dog was the ring bearer and so on. It's all made up, so couples shouldn't get in a tizzy over what's "expected." There is nothing but people's own ideas to say that a flower "girl" can't be a boy, or a ring bearer a grown woman, or the bride's best friend of either sex can't walk her down the aisle....
Let your sister ask your son and daughter to participate in whatever way, with whatever "title," your sister likes. If she doesn't care what the guests think, neither should you. It's lovely if she asks them to take part in any capacity. An 11-year-old, by the way, can do a lot of things besides be IN the wedding -- he can be in charge of the guest register and greeting people at the register; he can be in charge of decorating the "getaway car" if the couple is going to do that; he can even do a reading in a ceremony if the couple wants and he's good at getting up in front of people. Weddings are not carved in stone, and a creative couple can find many ways to include kids -- or not, as they choose.