Worried about step son

[Stepmoo]

My partner has three sons with his ex wife. I started getting to know the kids 2 years ago and they have been living with us part time (every other weekend and some weeknights averaging 3 nights a week) for almost a year and a half. The youngest is 3 years and 8 months.
He is a very cute little boy who I can be a delight, especially when on his own, but he still has lots of tantrums over not getting his own way. He's not good at sharing or playing well with his brothers, he's rough and naughty but very good at playing the baby when things go wrong, especially when they're his fault. Yes he's not even 4 but he's very aware of how to get away with things as 'the baby'. He's the toughest of the three boys for discipline and listening. All three boys have been known to rough house and play fight but the youngest always take it too far. He does also lash out with hitting, pinching and kicking his brothers outside of play fighting as well. They're more often getting hurt by him than him by them even though he's the smallest.
The thing I am most concerned about is his lack of speech. Only maybe half of what he says can be understood by us because we live with him (he probably wouldn't be understood at all by strangers) and that's only a word or at most three put together, not sentences. The rest is still just babble. He is way behind the 3 year old speech milestones and he is nearly 4. He has had a hearing test that came back fine, that his hearing was actually very good. The nursery have only just said they will refer him to speech therapy and that has not been arranged yet. But they don't seem that worried. We encourage his speech all the time but he just doesn't seem to be progressing. He has made barely any progresion with his speech in the last year at all. When he and his brothers first started living with us part time he seemed to be on the edge of a speech break through, trying to say new words all the time but he's not really moved on since then. Yes there was an adjustment period for all three boys living here some of the time but all things considered it smoother than anticipated. The youngest seemed least effected, he hasn't really very known his parents together and actually had a much stronger bond with my OH than his mother. I just feel with moving nurseries, he's at a different one now from last year, and all the time off because of covid he's not progressed and fallen through the cracks. No one seems to be worried enough or doing much about it. The health visitor initially said he needed a hearing test just in case as his speech was coming along slowly nearly 16 months ago!! He waited to have a test but after he did they said it was unreliable as he had a cold at the time. They were waiting on getting him another one when covid hit and everything got shut down. He finally got another test done a few months back and they concluded he had perfect hearing. I've said to my partner a few times that the youngest needs extra help with his speech and that his behaviour is probably linked to his acting out because he's not understood but he has always had a "all kids are different and develop at their own speed" attitude, which I understand especially as his oldest is ASD. But when you see the youngest with his peers you see how far behind he is. My OH has more recently admitted that the little one is falling behind but doesn't struggles with communication with his ex and doesn't know how to tackle it. His ex seems completely un-bothered. I am not a parent myself and in being a step parent I will inevitably be condemned for criticising the mother but I do feel she likes keeping him as the baby. She says to him "you're mummies baby aren't you, goo goo gaga" and encourages him to do the "goo goo gaga" bit back and when he does she laughs and he does it more. I maybe thinking to in to it but she's just encouraging him to be a cute baby that speaks nonesense. She should be encouraging actual speech not laughing and telling him how cute he is when he says goo goo gaga. She's insisted on waiting to potty train when he was ready before. We had nearly six months of him insisting on being changed every time he filled his nappy, those are the behaviours of a child that's ready. She doesn't want wean him off his dummie and muslin but again, he only has 9 months until school, he can't take it to school with him, they need to at least try to get him off it.
I'm at a loss. I spend a lot time with these kids and do a lot with them, I bath them and make lunches and I'm involved and I care. I'm not their mother and I am in no position to tell anyone how to parent but I'm concerned about things that I feel powerless to do anything about.

[JingleJohnsJulie]

I think there's a step parent section on MN where you might get some more responses but I think you've knock EF the nail on the head. As stepmum you're doing half the care but don't have a say in what's going on and frankly, his own DF doesn't seem bothered enough to do anything.

I'd step back a bit and let them get on with it.