I have a 6 year old daughter and a 20 month old daughter, so I'm kinda in the same situation. My oldest likes to control her sister too. I think it may just be a mothering thing. My oldest has always been around older children. She was the only grandchild on my husbands side, and the second on my side (her cousin was 6 when she was born). I see the same controlling thing going on all the time. She is constantly playing "on" her, not with her. I just tell Mckenzie that she needs to give her sister room. She is a little girl and want to explore things around her. Don't get me wrong, Karlie loves playining with her big sister, when she's actually playing with her. Mckenzie loves to take toys away from her, yell at her and that kind of stuff. She has not hit her, thank god, cause she knows what's going to happen if she does. Honestly, as long as Karlie isn't crying, most of the time I don't mind. Once Karlie starts crying, then I take control of the situation. Usually I send Mckenzie to her room to play with her own toys, not to punish, but to give them separate time. Sometimes it works, sometimes it doesn't. I think as long as she's not hurting your baby, then just let them play. Once it gets to the point when it starts annoying the baby, then you need to cut right in. Also, with the oldest being your stepdaughter, does you husband get involved when he's home? That may help as well. Hope this helped, sorry it was soo long!