Hello my dear @rosnely :)
According to the research I studied at the age of 2-3 is when children's aggressive behavior peaks. So, there is nothing abnormal about your 3 year-old. However, she need to be informed from now that this is not correct (even if she appears not to pay attention). Also, every time you see her being affectionate towards her sibling, reward her with a kiss or a complement. This will help her to understand that is the proper way to behave :)
As for looking for a counselor now. I think because there is nothing particularly abnormal in her behaviour, it is not necessary at the moment. In case she continues to present the same sort of behaviour at the age of 5-7, like Lilly, then you could consider professional help.
I wish you a great weekend too! :*