Tonks knows that Remus is - in a lot of ways - a broken man. Kingsley's told her all about Sirius' death, how Remus took it. And she saw some of the aftermath herself, after she was released from St Mungos. For a long time he's only a shadow of himself, an automaton working through Order missions, a ghost. And, Merlin, but it hurts to see him like that, seems so profoundly wrong and she knows that this must be what he was like before, after the Halloween when everything fell apart.
She wouldn't say that she's in love with him, though she supposes she must be and suspects that Molly suspects as much. Mostly, she feels achingly protective of him, so much so that it burns and leaves a hollowness in its wake. And there's no use for it, because he loves Sirius still and it's carved in every drawn line of his grief-stricken face.
It's her fault that Sirius is dead. If she hadn't been knocked out of the fight, then he wouldn't have had to intervene with Bellatrix. Every time she thinks of what happened in the Ministry that night, it's all that she can remember - her fault that Sirius is dead, and her fault that Remus is so broken.
And, still, she can't help the way that she feels. It's been there for too many months to be crushed in a moment. She's tried to clamp down on it, tried to focus on other things because she knows that there is no possibility of anything happening there, or at least not for a long time. But she can't.
She knows that they were more than friends, has known that for a long time, ever since one evening when she arrived early for an Order meeting and found them cuddling in the kitchen, Remus planting soft kisses to Sirius' forehead while Sirius methodically emptied mysterious phials down the sink. She slipped away again, quietly, without either of them seeing her, and when she came back just as the meeting was kicking off, they were sitting in their customary seats, neither giving any sign that anything unusual had happened. And so she knows that they were far more than just good friends, knows it with a knowledge that burns deep when she dares to think of that evening.
Tonks understands that Remus is broken, understands that he can never love her the way that she loves him – if, indeed, what she feels is love – knows that Sirius will still own his heart even in death. But she finds that she can deal with it, can accept it. And for her that's enough.