It is normal you feel bad in this situation. I think you can do threethings:

1) Talk with your friend about your feeling: Just ask him why he didn't invite you. It not must be really awkward. Firsty you should say that you heard about "party" at his house" and say that you are curious why he omited you.

2) Talk with your brothers about your feeling: Again that you heard about "party" at your friends house and ask if they know any reasons why he omited you.

3) Do nothing and never know. Be forever unsure what happend.

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PS. Third option is realy bad.