Toddler hitting baby how to stop her ?

Emma

My 2.5 year old baby girl keeps hitting my 15 month old baby girl she also will push her down when she's on the floor playing an she will find it funny when she hurts her but at the same time I don't think she a 100% understands what she's doing 😕 she as been like this with her since she as been born as anyone else been though the same ? If so would be nice to hear your side 😊