Whether or not Deckard is a Replicant, I think it can be argued that Roy's altruism was genuine, and not simply a case of one Replicant protecting another. Unless I'm misremembering something from the movie, a Replicant can't accurately and quickly distinguish between a human and a replicant like the test does, they aren't wired to sniff out and grade empathy. Given doctored memories, they can't even distinguish themselves from humans without the probing and dissection of an administered test. Anyway, back to Roy.
"Proud of yourself, little man?"
"Not very sporting to fire on an unarmed opponent. I thought you were supposed to be good. Aren't you the... 'good' man?"
"Quite an experience to live in fear, isn't it? That's what it is to be a slave."
"I've seen things you people wouldn't believe."
"Time to die." (what's interesting is that this was said to Deckard earlier by one of the other Replicants, then in malice, now in earnestness)
He's asserting and relating to Deckard like he's some different manner of creature altogether, someone who doesn't know the Replicant constant of persecution, of running, of being submerged in perpetual fear. If Deckard is a Replicant, Roy doesn't seem to know it, he regards Deckard as being as alien and as misunderstanding as any other human being he's met. Even in death, he makes no reference or allusion to Deckard's nature, for him it seems this moment is all about sparing a life when he could have felt perfectly justified in taking it.
Even if the two of them are still Replicants, it's still a beautiful moment because Roy shows he understands something significant about the value of life given (and not taken), and it's something Deckard is only beginning to understand. He in essence closes the movie by paying it forward by taking Rachel under his protection, and even if he turns out to be mistaken in thinking that they are fundamentally different beings (as Roy may have been with Deckard), that emerging understanding about the value of life is still powerful. It's a lesson learned by a replicant who knew what he was, then by a replicant who believes himself to be human, and by extension it is a lesson also for natural-born humanity, whose memories and lives were drawn into forming such humanized replicants to begin with.
Far from being a simple act of kin preserving kin, I think this moment is a delightfully convoluted act of compassion. The giver doesn't fully understand who he gives to or indeed himself, yet he gives. To be caught up in the discernment of who is natural-born here and who is engineered there is to miss the spirit of this exchange. It's an act of charity that transcends all boundaries: replicant, human, and everything in between.