Sister is allowed to tell her kids "you will not receive a free ride from me after you turn 18", but she's not allowed to tell them "I control your interactions with your extended family after you're adults".
If my kids are still living with me after they're adults and have graduated from high school, I expect them to contribute to the household, but the bar for kicking them out is going to be along the lines of "it's now been a couple of years; you're making no effort to find a job, even a fast-food or retail job to bring in a few bucks; you're not attending school; you're not taking a daily or weekly household task off our hands". It's not going to be "you've turned 18; pay us market rent or go on the streets." (And if one of my siblings or one of their older cousins wants to take them in? Great!)
no subject
If my kids are still living with me after they're adults and have graduated from high school, I expect them to contribute to the household, but the bar for kicking them out is going to be along the lines of "it's now been a couple of years; you're making no effort to find a job, even a fast-food or retail job to bring in a few bucks; you're not attending school; you're not taking a daily or weekly household task off our hands". It's not going to be "you've turned 18; pay us market rent or go on the streets." (And if one of my siblings or one of their older cousins wants to take them in? Great!)