wow, that's just horrible! I would be so hurt too! I wish I had some good advice for this one, but I'm not sure I do. I would like to think that your bro is just looking out for you as his sister, but is going about it the wrong way. You are very smart by not telling your FI about it, I'm sure it would really hurt him...
Hang in there girl! I'm sorry I don't have any good words of wisdom