DD2 (17 months) is going through a phase when she thinks it's fun to pull her older sister's hair, pinch her etc.. It's usually not for any obvious reason. DD1 (4) does not retaliate - she just cries 
Looking for advice on what's best to do here - we have tried saying 'no' (obviously), trying to encourage DD2 to be gentle etc.. Should I remove her from the situation to try to make her understand that she shouldn't do this? It doesn't seem fair to just say to DD1 that I can't do much about it, as DD2 doesn't understand...Any advice welcome!