[The most obvious scenario is the correct one, and Albert does have the presence of mind to be slightly shamefaced at his response. He knows it's taboo, of course. He knows better than anyone.]

...William, primarily.

[The rest of the story becomes important now, the secrets Albert should take to his grave, but that Ignis' eyes are going to ask.]

They're... not really my brothers. Not by birth. They're adopted. And William isn't his real name.

[He looks down, anywhere but at Ignis.]

I didn't tell you everything. I wasn't sure how. If I tell you how we came to be brothers, you may look at me less kindly.