Afaik using the same IP on different accounts is not allowed because it can be a sign of abusing their games and it is written on their
terms that if they think that you guys are abusing they have the rights to ban your accounts. You and your brothers should've use a vpn to avoid using the same IP.
I read the terms, it doesn't say anything about using the same IP, even multiple accounts are ok in the rules. It says its prohibited to use multiple accounts to play. It wasn't the case as I explained.
Conversation with a mod:
https://postimg.org/gallery/2w3362t46/Best phrases: "rule is, I can bend the rules. I did. You didn't get your way. lol.", "you think I'm gonna check your book? I believe you're a cheater and that makes you dishonest.".
I need to say anything else?
Same IP means same person on internet because it refers to the same address (though not always true). If you wanted to play seriously while online, hence it's better not to share your connection with others even though they are your 'brothers'.