Complex sentence: Evidently, the post was closely associated with the imperial family, as affirmed further when Stilicho was appointed to it on the occasion of his marriage to the adopted niece of Emperor Theodosius I (r.378--395), Serena.
Split into simpler sentences: Evidently, the post was closely associated with the imperial family. Thus, Stilicho was appointed to it when he married the adopted niece of Emperor Theodosius I (r.378--395), Serena.