Some white noise in the room where her little sister is sleeping could help buffer the problem. If you are less stressed about the chance of the baby waking, you are less likely to be tense about "stopping" behavior (which is a resisting of them that leads to counter-resistance on their part and makes the behavior more likely.) There are cheap white noise machines/clocks in addition to other household options.
You can go on encouraging a shift or change of course, but if you are not as tense/upset (because of the white noise assistance) you won't be as focused on the negative. Whatever you focus on (and if you're all about "don't scream" or "stop making noise" or "that's too loud," then you still are focused on screaming, noise, and "loud") you get more of.
The main thing is the shift in you (away from resistance), but I know that can be really hard if you are stressing or feel powerless, which tends to make you more angry.
at 5:51 AM on May. 20, 2011