sibling jealousy?

hi. I am really worried about letting my girls share a room anymore. My girls are 2 last october and 1 in less then 2 weeks. Today i put them to bed the way i normally would but i put my youngest into the portacot instead of her normal cot as i was cleaning her mattress. I heard my youngest screaming and walked in on my eldest sitting on her and covering.her with the toys. I got her out and put her to bed again. I soon here my youngest screaming again and find my eldest in the cot again and smothering my youngest with a nappy. I have no idea what to do or why she has all of a sudden started this. They normally do it when the baby is first born? Has anyone experienced anything similar? I don't know what to do as they share a room and i only have a 2 bedroom house and don't want to do? Any help would be wonderful
Tia