[Both of them curling in closer together has Sirius doing the same, wishing he could just hold them both and protect them from the world. Wishing, desperately, that there was something he could do-- Some way to wave his wand and make this war be over so they don't have to be so scared.

He's coming up empty, for now, and it's frustrating. It's got his laugh a little sharp and shaky in response to their words.]

Suppose it'd be-- a bad idea to make you cross, right now. Guess I'll try to be good.

[It's not a promise. How can they possibly promise, right now? They couldn't even keep themselves out of this situation; how could they promise they'll make it all right out to the other end of the war? It's absolutely terrifying to think about.]

--Merlin, I love you two. So, so much.

[Saying it helps make things feel slightly less terrifying. Just slightly.]