Or was it the song from the concerts/character's song or something like that? Because if yes, then I think there's one titled "Oretachi wa Family". Sorry, I thought you meant Opening/Ending hahha :p
Wow, has this community died or something? I've just joined! Lol
Anyway, just wanna leave a REALLY sweet family/romance fic about them. (But since I cannot leave links yet, I'll just give the name of thr fic)
Thoughts Read Unspoken by Qwi-Xux.