Has the older one been integrated into the flock? Or is it separated from the flock just with the mom? If it's just the mom and the 6-week-old, that's not a good setup to add babies to, especially if they're all in a small space. The teenager is going to peck the crap out of the babies, and the mom will want to protect them and peck the crap out of her weaned teenager. The poor teenager will have nobody to socialize with and will get stressed. If the teenager has been integrated with the flock, then you can give the broody babies and things should work out. How long has she been broody this second time? Don't add babies until she's been sitting for about 3 weeks.