My youngest son will be 3 in January, and he is a turd. I'd be convinced he didn't like me anymore if I paid any attention to the fact that he seems to get pure joy out of causing me physical pain. I too am the only person he does this to - he is an angel to daycare staff, can be a turd to his brother and sister but they do it back, doesn't try it with his daddy. There are times he is an angel too - but most of those are when he is sleeping or sick. He is rough and tumble, and I am not - so he tries to rough and tumble me, and I don't do it back... which ticks him off.
I'd give it up for a while on the whole "acknowledging his brother" thing... mainly because you're creating an issue that doesn't exist, and doesn't need to be there for the sake of your marriage.
Here's how that goes down; your pregnancy hormones make you uber sensitive to everything, and you want everyone - including a self absorbed two year old - to be overjoyed at this thing in your belly that you and his daddy both seem to be paying more attention to than him.
Then it comes out - and rather than being able to treat it like a toy that could stand up to some boy playing, he can only be 'nice to it'... and you and his daddy just think it's great, to him - it's got the same relevance as a squid - he can't play with it. But you get upset because he doesn't 'love' it, and you think he's 'bad' and he gets unhappy... and feels like everyone loves the squid more than him.
So you start feeling like the wicked step mother because of these hormones, and the normal developmental stages of a boy and the fact that he is your "STEP" son - as opposed to your real son; and you start putting things up to your husband as he treats his son better or differently or has different expectations for in comparison to the son you share.
And then it's his son versus your son.
And then he'll have them both every other weekend, with their new stepmom... and that sucks, from what I understand.
My husband had children from his first marriage when we met, and his ex wife is the daughter of satan - she did everything she could to convince those babies that I was a harlot who made their daddy leave them, despite the fact that she had a string of live ins between the days before he left (yes, really) and the day the divorce finalized - and he and I met the month before everything finalized (and his children met me 6 or 8 months later than that)... we went through the usual getting to know you things (they were 4 and 6) - and we married a year later. When I got pregnant, my dd was thrilled - my ds (7) was worried that he was going to be usurped. His mom told them both that their dad wouldn't want them anymore after he had another kid. It took six months to debunk that myth and - by the time the baby was 1, he was everyone's baby. Between the first and second, our oldest went through some more stuff with meeting his biological dad (not my husband), and anger issues - and decided not to come over anymore for awhile. We let it happen. Invite him regularly, but don't force anything... he's started coming back around, mainly to see his little brothers - but also to spend time with his dad, and to get help with his homework from me (because he still thinks he can't love me and please his mom at the same time). Our 11 year old is more a girl at heart - she enjoys being a big sister, although she still runs into some mom house - dad house issues with different rules and expectations. It is what it is - and what it's not is a love issue. Don't let it become one.