Translation:This is the man whose brother has two children.
I think dette doesn't really refer to a known wors at this point in the sentence yet (ultimately we know it refers to manden, but I think we should ignore that because manden has not yet been introduced in the sentence or something), and so we should default to dette. Or something like that. It somehow feels intuitive to me but I don't know if I rationalised it correctly.
Could anyone tell me which one was the lesson about "this" and "that"? Thank you!