Pregnancy/ future step kid

Hi everyone, so my boyfriend and I are expecting our first child in May. Every other weekend we drive a state over to see his kid from another relationship (he is 2 years old).

My boyfriends kid (let’s call him Kai) stays with my boyfriends grandma a few times every week, and every other weekend. I used to enjoy going to her house, to see Kai and get closer to my boyfriends kid. Now i feel guilt and confusion as to why every time he throws a “terrible twos” tantrum i find myself rolling my eyes and wondering why no one is disciplining him. I find myself wondering “why aren’t they teaching him to be respectful when he’s young”. I get hit and stuff thrown at me every time i go over there, and spit on. When i mention to my boyfriend that it hurts my feelings i get told “he’s only two, he doesn’t know better” but I’m getting bruises and getting stuff thrown at my stomach while I’m pregnant.

I know his Kais mom isn’t the best and she doesn’t seem to really care about him, and my boyfriends grandma spoils him to no end and won’t even let my boyfriend or anyone else tell him “no” when he does something bad. She’s bipolar and gets angry when you do the slightest thing wrong. I feel like I’m going to eventually make a terrible step mom because i have no understanding for Kai, and i just feel like “well he’s not my kid”. I feel absolutely terrible for being and feeling this way, so i will bring him food, cuddle him when he’s sad and so on, but then i get hit/ spit on for trying so i just cry and walk away. It feels never ending these weekends, and i always dread coming. I only go so my boyfriends grandma is happy and my boyfriend doesn’t think i hate his kid. But i don’t know what to do anymore, I’m writing this after Kai just got mad at his toy, and came at me with a raised fist and smashed into me purposely (when i was all the way across the room). It makes me wonder if i can even be a parent with getting so mad at stuff like this ......I just need advice :(😞😞😞