Few questions regarding the rules

He can use the site, but he will not be able to enter the SGTools protected giveaways that have a check for this specific rule.

He should be fine.

By the way, if you live at the same location, you should let support know that your brother has an account on the same IP. Otherwise you might risk getting both permanently suspended for multiple accounts.

If you enter a giveaway for that particular game again in the future and win it, and you activate it the second time, you will get banned again for multiple win, assuming your old win (that you were banned for the first time) is still marked as received. This happened to me (over a $1 DLC that I forgot about) and seems ridiculous, but support will have no mercy since it's "2nd offense".

To avoid that you must avoid entering giveaways from that game ever again. This means that you can effectively no longer win this particular game from this site - if you want it in the future, you must buy it and activate it yourself. I would advise you to buy and activate it either way, since you will then avoid the SGT barrier for all future SGT giveaways as well. (Also, now that you've bought it and own it, SG will stop showing you these giveaways after your next sync, which prevents you from accidentally winning it again in the future.)