OP, ignore anyone obsessing over your choice of words like “horror” or “dramatic.” Jesus Christ, you are a new mum, your hormones are wild, and that is just how some people speak. You are doing a tough job and handling it the best you can.
You are not a wicked stepmother for not wanting a child picking up your 4 week old. I was a nut case at that stage myself. Anyone piling on you for being protective has just forgotten what those early weeks are really like.
Step-parenting posts always seem to bring out the angry ex wives club, bitter and judgmental, lecturing like they are morally superior while conveniently forgetting that they are basically doing for their own kids exactly what you are doing for yours, protecting them. And just because back in the day 12 year olds were left to parent siblings does not make it normal, that was DIY childcare and thankfully we have upgraded.
bonding can happen without any holding involved, there are gentle safe ways for her to do it without holding the baby. Talking to him while you hold him, helping with bath prep, or holding black and white cards during mat play, the list goes on. My baby thinks it’s hilarious when I do a full rendition of early 90’s pop music whilst she just sits in her bouncer. She probably just wants to be involved, and you can guide her, I would teach her the importance of holding him properly though just for when he’s older or if, as children do, she wants to attempt to pick him up again anyway.
Your baby, your rules, and no bitter internet ex wives club gets a vote.