hmmm... okay here goes...
1) That's great news that you have the family approval!
2) don't be offended that he called someone fat just because you've been bigger at one time. That's just an unnecessary stress. People can be mean and it's obvious that he didn't like her for his brother so he could have easily said anything else like that she smelled bad or was annoying. In all likelihood you wouldn't consider that offensive to you.
3) You were called thin! live it up!
4) It's been 2 months, you're allowed to let it go now.
So ya, take it as a compliment that your boyfriend's brother likes you and that you are better than his last girlfriend and of couse everyone before her too.
Because that's always a nice boost to the ego.
I still remember when I first met my fiance's friends one of them said "You better stay thin". Lovely eh?
I just took it as 'sweet! I was just called thin!' lol I totally ignored the negativity of the message because why should i concern myself with that.