Can a YOUNGER sibling be a legal guardian if the older sibling has a disability?

I'm 20 and my brother is 23, but he is autistic (not severely but still enough to require care). Anyway he frequently stays with my boyfriend and I so he might as well be officially under my care. I'm an office manager on a good wage, and my boyfriend is in a stable career despite being two years younger than me, so we could easily support another person living with us.

But yeah how do I go about doing this? pic unrelated