Sirius viewed Cyrus and Grace as more his parents than Orion and Walburga ever were. He was bereft when Cyrus died. So yes -- they were close.

The hunger for love and acceptance can lead people to dark places, sometimes, but I don't think that'll happen with Terry. I'm a bit more worried that the thirst to prove to himself that he deserves that love and acceptance will drive him to take risks he's too young and inexperienced to properly manage. All any of us can do is to keep telling him that our care is not conditional. And I'm sure he doesn't see it that way. From the inside, it feels like he just wants to be useful, to do the things that are within his power.