A little advice

Hi my dd ellie is 6 in august and is wonderful with her little brother, but leo is 7 months and much more sturdier now and when im not looking she has started picking up off the floor and swinging him around sitting on the sofa and leaving him, he is just to heavy for her to do this and she struggles and as had a few near misses, every time i try and explain to her about she isnt really big enough and strong enough but it doesnt seem to make a difference i end up shouting at her as she gives me a fright, i dont want her to think she cant touch him and become resentful i just dont know what to try next any ideas or advice would be much appriciated as im going to have another lo in 16 weeks im going to need eyes in the back of my head.
vikki xx