Looking for some gentle feedback

I'm a first time mom, but my partner already has a child. My daughter is 1, and I had her at 31, after a lot of life experience, earning my master's degree in clinical social work (specializing in work with children and families), and establishing my career. My partner had his first at 20, and separated from his partner a couple years later. (By no means was this a mature split, and while I accepted my partner's perspective on it at first, the longer we're together, the more clearly I see exactly what his ex was seeing.)

Anyway, their child is a sweet, kind child...who has been raised with basically no boundaries, and has no skills for self-regulation. I'm not sure what it's like at his mom's house, but my partner is honestly pretty disengaged and lazy, and sees upholding boundaries and expectations as too much work (which is very selfish).

Now before anyone says "you clearly have different views on parenting, you should leave", consider that if I leave, my 1 year old will spend half of her life in chaos, which is a last resort option for me. Before I moved in with partner, his toddler ate only candy and pizza rolls (no joke), was up whenever my partner was (often going to bed around midnight), coslept (despite frequently peeing the bed), and the house was a mess (toys, food, filth just everywhere).

"Why did you date a guy like that??" Well, he cleaned before I came, so I thought he was keeping up house beautifully. I also knew toddlers can be picky and challenging, so I dismissed concerns around no bedtime and some food choices. That sort of thing.

Anyway, I can't risk that he would revert to that sort of "parenting" if we split, and there's no way I'd get more than 50/50. He just used our daughter to ensure he gets at least 50/50 custody of his son, so I'm sure he'd do the same to me and get the same results.

Probably nobody made it this far, but the issue here is that now that my daughter can walk, I'm struggling with how to facilitate interactions between her and her brother. My partner would have zero boundaries if it were up to him, so he sees me as being hyper controlling and trying to keep the kids separate. I'm definitely not intending that, I just think it's important to teach respect for body and belongings early on: if you're scratching your crotch, you shouldn't immediately play with toys your sister is going to put in her mouth. If she's playing with something, clearly not responding to you, then you can go to her or play independently until she decides to; it's not respectful to get up in her space because you want attention when she doesn't.

Am I crazy? I've struggled with PPA/PPD really badly, and to be completely honest, I have absolutely no one to talk to about it. My therapist just constantly validated me (not helpful), my sisters shame me, my friends don't have kids or stepkids, my partner gets really reactive about anything involving his kid... And I can't bring myself to fess up to my mom that she was right and I never should have gotten involved in this situation. So I've got nobody, apart from Reddit