Role play as I always say is good. Try and remember that she's not sitting there conjuring up ideas on how to hurt him. Nor is she going to be the next dexter she's learning. I would get two of her favourite toys and demonstrate being gentle vs not gentle. Remind her constantly and make a big deal when she is gentle. Positive reinforcement is always more effective than time outs etc at this age. Because its about teaching her what to do instead. And be on her case about it. If they are in the floor together teach her by asking her to get a toy for DS. Ask her to help out as much as possible. Little girls love being little mummies. And try not to set her up to fail by leaving them to their own devices until you feel she understands. I know this is hard. Has he got a little seat that he can sit in when they play together. And if she's playing with something that she doesn't want him to get into set her up at a table above his reaching level so that she isn't going to get frustrated with him. And worst case scenario is what I used to call the logie performance. Where if she hurts him make a big deal out of it. Don't scream and tell. But gasp and make out like she could have really hurt him. I know it sounds really manipulative but its how I was able to stop both my two from doing things that could end up hurting themselves (like touching a hot oven etc) yelling doesn't work and I never smacked. But when you express an exaggerated emotion they tend to understand it more than just time out/yelling etc. Because it touches on their ability to feel empathy. And they realise the seriousness of the situation. At that age a little one on one chat is really not effective. Repetition is the key, positive reinforcement and helping them by showing them alternatives. And give her a voice if you can see she's feeling frustrated. So if you can tell she's finding her brother hard to deal with tell her you understand that it's hard for her. Give her alternatives to help her feel less frustrated and tell her she's an awesome big sister by being so patient. And even though he's a baby if he's being unintentionally rough remind him to be gentle so that she understands that he is only learning too. And she will start to mimic how you deal with it. And for you, it's totally normal to feel frustrated by it. You're doing great