I have to disagree with both of you -- while also partly agreeing at the same time. I think the source of this apparent confusion is that we're discussing at least two different things in parallel here.
The two words, "morals" and "ethics" are often used interchangeably, but I suggest that in order to avoid confusion, we follow the convention of using "morals" to refer to that which is internalized by the individual (and which, I have argued above, is based on what Hume called "sentiment" and can be obtained intuitively), and "ethics" to refer to that which is dictated by social convention (and, being based on rules, must be learned -- as in professional ethics, political ethics, etc).
Kin altruism is arguably not a product of either, but a basic biological drive -- and as such, is often the cause of moral or ethical dilemmas, rather than a solution.
As for mathematical (or moral) Platonism, that's a puzzle I never could quite manage to solve, and while I don't see its solution as absolutely vital to answering the question at hand, it would be great to see it solved anyway.
But that's four separate discussions.