First off, the thread title is misleading. Nothing about this incident is related to the reliability of the site.

The nature of giveaway competitions is such that all judges decisions are final. You've admitted already that your brothers sometimes play with the same IP, and this is already in violation of the "multiple account" rule. You should have cleared this off with the Mod before entering - and they would have most likely disqualified you from the contest. Playing with your brothers would have meant some form of collusion and given you an unfair advantage over others.

Most sites anyway cover single account as per person, per IP, per device, per household. If anything is in doubt, point 8 is the binding one: "We reserve the right to change this giveaway at any moment, and to exclude people from the giveaway if we feel we need to do so."