There was a fic I read not long ago that i forgot to save, NormalAU, Dean is a lawyer and Cas is a social worker and they meet when Dean puts away the mother of one of Cas's charges, who bonds with both of them. the kid is a young teenage boy who gets sent to foster care and at one point runs away but then Dean and Cas start spending time with him, and they get together, and despite their relationship being so new they decide to adopt him and John wasn't happy about it thought it was happening too fast(and he had never really been thrilled with Dean being Bi) but he comes around. and Anna is Cas's sister or cousin but his only close relative. it might have been separated into several fics i'm pretty sure.
Thanks in advance.

Found: link in comments, (sorry about the mix up with the tags)