What real-life artist draws Sagiri's art in the anime?
Ah, it's Tiv, who tweeted about this here:

アニメ「エロマンガ先生」、紗霧ちゃんのお仕事に協力させていただいてます。これからもしばらくよろしくお願いします！#エロマンガ先生
I'll be helping with Sagiri's work on the anime 'Eromanga-sensei'. [usual Japanese pleasantries] #EromangaSensei

Tiv is probably better known for being the artist for the Masamune-kun no Revenge LNs/manga; the resemblance of that and her other art to Sagiri's art is unmistakable (see e.g. her portfolio website &quot;Planova&quot;).