Paternal age and child outcomes redux

Another article about father's age and child outcomes was profiled in the Guardian by science writer Ian Sample with quotes from the lead author and from me and Jenn Roff at Queens College.  We think this research is interesting, but we're unsure whether the statistical strategy, which examines siblings rather than similar kids in different families, can plausibly separate the effects of paternal age from birth order.  Are junior's test scores lower because his dad was older when he was born, or because his older sibling absorbed more parental time?  Or did the parents get divorced?