apelilae,
I was hoping that someone else would answer this because I'm not sure if what I have to say will be too helpful. But, since no one else has answered, I'll give it a shot! (Or maybe a stab in the dark.

)
I'm going to guess that Liam's behaviour is new brother related but of course I could be wrong as it does also sound like very typical 2 year stuff. (And I bring up the new brother stuff a bit hesitantly as I remember when my #2 was born it seemed than any change in #1 was always blamed on the new arrival. He doesn't want to put on his jacket? Must be the new baby! What? His poop looks different? It MUST be because he has a new sister. He fell down the stairs? Well, clearly that's a result of his new, sibling-related, inner turmoil.)
But, I digress. One of the things I learned with my two is that the older child can be very happy to have the sibling and very loving toward the sibling but still be having a problem adjusting. In our case my son was really happy to have a new sister but after a month or two started to develop what seemed like conflict with her. He would poke and push her and the more I told him not to, the more he did it and the more violent it became. And, to make a very long story short, I finally realized that he was not mad at her, he was mad at me. She was purely a way to get my attention. (Maybe this is obvious to others but I always anticipated him being angry with and resentful of her not me.) Because it was an issue with me that he was having, anything I did to try and mend his relationship with his sister was not effective in changing his behaviour towards her. It was only when I treated it as an issue between me and him that I could make any progress in fixing things.
So, my only idea on the Liam situation is that maybe he is happy to have a brother but that he is having a problem with something that changed when his brother came along. In our case it was a change in the relationship with mom but it could also be a more situational type change. For instance, maybe he is happy to share his life/toys/love/mom with Seamus. But it's been a BIG emotional adjustment and he's all "shared-out". There's no emotional room left for sharing beyond his immediate family both with toys and affection.