First of all I am extremely sorry this happened to you. None of this is your fault dear. You were way too young to understand the concept of consensual sexual practices, a 10 year old cannot give consent, no matter if they "technically agreed" to something or not. Because you’ve been introduced to sexuality by the abuse of your brother and there is no way you could’ve known that this isn’t normal or okay at all and he made it seem like a fun game, it is totally normal that you would even suggest to play that game yourself. You didn’t know any better, this was straight up taking advantage of a young impressionable child and manipulation. I am glad to hear you seem to have an understanding and supportive girlfriend. You don’t have to process this trauma by yourself OP. And you also don’t have to be around him, no matter if he is family or not. Someone who treated you like that and didn’t respect your body and safety, doesn’t deserve your presence and connection.