Any advice for a terrible two's

How about the atmosphere in the house. Is there anything that causes her to behave like this?

Is your little one getting enough to do that challenges her mind? She could be bored and looking for more interesting things to do?

Your girl is so good and sweet. How old is her cousin? Firstly, I'd think about the environment, as children always like to learn from others. Did she see anyone do that before? I personally do not like violence at all, but sometimes it seems like it just comes quite naturally for kids, especially young kids. They may not mean to hurt others, perhaps they just want to do something they want to do or don't like something others are doing, but they are still too young to express it in words, so here come their hands or legs. When the children are at this young age, I'd hold their hand and show them how to use hand to pat on others gently, and tell them this is what we should do with our hands - love others or do things. Perhaps you can give your daughter do something on her own, so they don't play near to each other. Children at this age may not play well together yet. How about let them play with nature together? Do they play along well? I have noticed my children and other children, when they play with nature things (such as water, sand, flour, etc or in the park), they do not fight, as there are plenty of things to play together with. My children are very active all the time, so I guess it's quite normal for some children.