I'm late to the post, but I thought I'd make a couple of comments. Firstly, this is sad:
He said he'd noticed it but it's best to ignore it because his parents and brother are always trying to take stuff away from him. Three points:
What's happening isn't a surprise to your boyfriend. It's been an ongoing pattern of behavior his whole life. He is now watching his family try to take you away from him.
The way he has coped with this in the past is to ignore it. I suspect that in the past he may have stood up for himself and faced repercussions (three against one aren't great odds). The current situation is different. This doesn't just affect him. It affects you. And while it is his family you have a right to defend yourself. It's now three against two.
I suspect that your brothers family always believe they are right. And this is going to be your biggest obstacle. They are unlikely to stop without some form of intervention. Hell, even with intervention they may not stop.
This is brazen though:
his mother asked me to take a picture with the brother and posted it on Facebook… but not one picture of me and my boyfriend together. And the caption of the picture of me and his brother was "an absolutely adorable couple, love you [my name]!"
The above is taking subtle to pushy. Let me ask you something, if they continue this pattern of behavior do you think this relationship will last? I think the answer is no. What is now an uncomfortable issue will become a major issue and then a break up issue.
You want to defend your boyfriend. So defend him. Talk him up. Let them know that you believe your boyfriend is above his older brother (keep in mind, his family sees you as an equal). You can do this cleverly. You can do this overtly (for example your Facebook response to your boyfriends mum - classic. And good for you for standing up to the mum). And if you want, you can limit contact with them.
Because remember, this is your relationship. So, do what you got to do to protect it. I wish you the best in the future. I hope it all works out. I hope to read a 'happy' update.