I don't think anyone else could really know what his parents' perspective is besides his parents... but it seems like they just don't care and that is wrong to do to the poor child. The child is going to feel left out and insignificant, if not already. It is totally not wrong to be mad at them...you might feel like you might want to talk to them about this, but sometimes you have to think if its worth getting involved. Maybe YOU and your husband could do fun stuff with the child, if you don't already, and just help the child that way, and maybe also talk to him about stuff, just so he doesn't develop a negative mindset about himself and what he is worth? But its totally up to you if you want to talk to his parents about it or not. You are not wrong in this situation.. because he is also your brother-in-law and he is family. Hope this helps, stay strong!