I want to gain a "Older Brother" relationship with someone I know. But there's a problem. Help!?

Talk to your friend first. Ask him what his aunt/uncle are like. But yeah, speak to them with your friend there. If you talk to the parents, their opinion is the only one that matters. I think it's really great what you are trying to do. Talk to all of them together so that you can get a plan together. Like if you can hang out once a week or do things together. The more you act like a brother the more you will be welcomed. So, start hanging with him like he's your brother.