How to get my 15 month old to be wary of her 6 week old sister?

My daughter doesnt throw much of a fit about having her little sister around BUT, if she sits on my lap in the recliner when Im burping the baby and gets mad while Im burping her shell slap the babies butt. Or in bed shell roll over and bang her head against her sister head. A little bit ago the baby was laying in my arms and she went to place her hands down to get a better look at my laptop and put her weight on her hand which was on the babies sternum. Its nerve wracking. When the baby is in her chair and shes in the room with her she doesnt pick at her or mess with her so its not as if she has anything against her or hates her or anything, she just doesnt really notice her and lives her life around the baby which in the long run is hurting the baby when they are together. and I dont want to not ever have them together because cuddling with them together is the only reason i got my recliner, any ideas on how to get her to realize that she needs to be careful around the baby?