This is how I imagine the two would get together after Sirius' death, assuming Sirius and Remus were together.
"We lost a great man last week. He was a friend, a colleague, a father figure to some, a brother, and so much more. We were all touched by Sirius Black, and the world is a bleaker place without him. Please, raise your wands in a moment of silence. To Sirius Black."
"To Sirius Black" came the reply, the dimly lit kitchen of the Order's headquarters had been illuminated by the wand tips. Remus Lupin held back a choked sob.
A soft hand enveloped his, and he looked up into light brown eyes, so familiar, yet different from his lovers. "Dora" he said softly, giving the girl a small smile. She was young, just a few years older than he was when he joined the Order the first time, but had a lot of promise as an Auror. Her skills were vital to their missions, and she had become a semi-permanent fixture in the house. She had small features that reminded him of Sirius, a softness of their eyes, the fullness of their lips, the boisterous, larger than life attitude that he could never get enough of.
No, don't think like that. He shouldn't, couldn't possibly start to think of Nymphadora as a replacement for Sirius, no matter how much the two may act alike. She was a child, barely out of school, and he was an old decrepit man. What would her parents think?
Not only that, but his mate had just died, the wolf inside was breaking at that loss. He was barely keeping it together. Most assumed it was just the loss of a best friend, much like when James and Lily passed. But this was so much more, and he had no one to confide in.
No one knew he and Sirius were together. That they had spent the last two years living here, not just as co-habitants of the Order headquarters, but as lovers. No one knew how each had pined for the other when they were boys, how Remus thought of Sirius every night while he was in Azkaban, conflicted about loving a man who was a murderer.
They had just talked about coming clean with everyone, letting their relationship be public. Some had assumptions, he was sure. Harry eyed them suspiciously when they came down for breakfast at the same time one too many times.
Sirius had just told Harry he could come live at Grimmauld Place permanently, and they were going to explain their relationship before that happened, but now? How could Remus ever tell anyone what they were? Taint Sirius's memory that way… No, he would keep this secret, but he had to move on. And there was someone so willing, so close, and so similar. Would the wolf accept her?
Remus wasn't sure if he even wanted to try at this point. He had gone through enough heartbreak, he wasn't sure he could take much more. And what happens when Nymphadora gets older, decides she doesn't want to be saddled to an old man like him? He'll be left alone, once more. No, he can't do that.
He tugged his hand away and turned his head sharply. "Dora, don't" he said, as he pushed his way through the mass of bodies paying their respects and barged into the room he had been sharing with Sirius, crimson and gold banners flying on the walls, lions perched on every surface, watching with beady eyes as he sank to the floor and sobbed.