Help with a toddler hell bent on hurting her baby sister

Today I've had to separate my 2y8m and 7 month old because the toddler is hell bent on causing pain to the baby. I've already set up a play pen as a safe zone and in the time it took for me to go to the toilet, the toddler had jumped into the play pen with a pillow, put it on the babies face and was sitting on it laughing. If I hadn't heard the baby scream something really terrible could have happened and I thought I had them safe but apparently not. Any further tips on how to stop your toddler trying to murder their sibling in small windows of opportunity.