Disciplining Step Children

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So my SO & I have been discussing my disciplining his son whenever he does something wrong or acts up and he isn't around to witness what happened or whatever. My thing is, I'm currently pregnant with our first and couldn't imagine someone else hitting my child or getting after them.. but I'm not quite a parent just yet so that may change when my daughter here is and actually running around wild.

What are yalls opinions/experiences on disciplining your step child? any advice would definitely be helpful. I love my step son but sometimes he can act like a little asshole but he's just too cute to get after & I feel bad, lol.

Lately I've been in that habit of just calling his dad when he does something bad but then he's like, "why didn't you just get after him? he's not gonna respect you if you let him get away with everything he does wrong."

Update — I must add that his son is 2 1/2yrs and I’ve been around since before he was born so he knows me quite well. My SO says if I don’t learn to get after him/correct him our daughter will resent me for disciplining her only and not her brother as well.

HELP!