Not really an AIBU but I am fuming and need help/calming down before I act (again)….but feel free to tell me if you think IABU if you so wish.
I was washing up in the kitchen while DD2 (3months) in her swing & DD1 (5) in the front room right opposite. I hear DD1 a couple of times say “and back down”, assuming she means the swing I pop my head back expecting to see them ‘playing’ as usual except I see DD2 slightly lifted by DD1 arms which she quickly removes sending DD2 flying back down in her swing. This is obviously what she was doing with the “and down again”, lifting her up and dropping her down!! I was so shocked/mad I screamed at DD1 and pulled her away to the stairs and told her I didn’t want to see her and she should go to her room. DD2 seems ok but god knows how many times she was dropped back 
. DD2 is our long tried for/awaited rainbow baby so maybe I was too much and overreacted? I’m not sure though so need to know how I deal with DD1 now?