They are not relatives. They just 愛 each other so much :D Lee is Gai's お気に入り student. If you're asking why they look a like, that's because Lee loves Gai so much that he made his hair like him and he always wears that suit which gai gave him.
While they do look relatively similar they are not related whatsoever they are just basic sensei (teacher) and student maybe even a bit もっと見る because Lee does view Guy-sensei as a role model but they are not related.
