Hi OP.
I would think it's something that's easily remedied by combining some helicoptering and zero tolerance.
From the outset, when mine were just tiny babies (and I mean babies), as soon as they attempted a slap or a punch, I would immediately set them straight down on the floor with a firm "NO! No hitting!" and walk away from them or turn my back. Of course, they would cry with shock...but they soon learned that hitting/pushing/grabbing/kicking would produce an effect that was negative only to themselves.
I never laughed at them OR lost my temper, I simply cut the behaviour dead. Then I repeated as neccessary.
You can try something similar with your dd. As soon as she strikes out, no matter how gently....you say "NO! No hitting!" and immediately remove her from the situation without another word. Engage again only when she has calmed down and is ready to say sorry. Do not enter into a dialogue about it...do not try to explain why you are angry (because you can do that later after everyone has gone home), do not respond to any pleas for attention or cuddles....until she can say "sorry". Then tell her she is a good girl to say sorry and give her a hug. Then it is over and done with. Till the next time.
Don't be drawn into "they are both as bad as each other" type of reasonings either. It matters not what other toddlers do....your only concern is what YOUR child is doing. Neither should you justify it as something that "all toddlers do" - they don't. Mine don't, owing to the consistency of the technique I have described...and I certainly don't appreciate other kids who do. Habitual thumpers are a pain in the arse.
It IS normal toddler behaviour, but it is NEVER to be tolerated, dismissed, or laughed at. Zero tolerance.